v6.0.0-rc.2 release of PowerShell Core

Here are the major changes:

* Breaking changes

- Skip null-element check for collections with a value-type element type (#5432)
- Make `AllSigned` execution policy require modules under `$PSHome` to be signed (#5511)

* Engine updates and fixes

- Update PowerShell to use `2.0.4` dotnet core runtime. (#5677)
- Remove references to the old executable `powershell` or `powershell.exe` (#5408)

* General cmdlet updates and fixes

- Remove unnecessary check for `Paths.count > 0`, in the `*-FileCatalog` CmdLets (#5596)
- Use explicit `libpsl-native` binary name for `dllimport`. (#5580)

* Build and Packaging Improvements

- Fix `Get-EnvironmentInformation` to properly check for CoreCLR (#5592) (Thanks @markekraus!)
- Make Travis CI use `libcurl+openssl+gssapi` (#5629) (Thanks @markekraus!)
- Disambiguate icon for daily builds on Windows (#5467) (Thanks @bergmeister!)
- Fix `Import-CliXml` tests which still use `powershell` instead of `pwsh` and make sure it fails if it regresses (#5521) (Thanks @markekraus!)
- Update port number used for WebCmdlets tests which broke due to a change in AppVeyor (#5520) (Thanks @markekraus!)
- Clean up use of `Runspaceconfiguration` from comments and xUnit test code (#5569) (Thanks @Bhaal22!)
- Replace `HttpListener` Response Tests with WebListener (#5540, #5605) (Thanks @markekraus!)
- Fix the path to `powershell_xxx.inc` in Start-Build (#5538) (Thanks @iSazonov!)
- Remove Pester as a module include with the PowerShell Packages.
  You should be able to add it by running `Install-Module Pester`. (#5623, #5631)
- Refactor `New-UnixPackaging` into functions to make the large function more readable. (#5625)
- Make the experience better when `Start-PSPester` doesn't find Pester (#5673)
- Update packaging and release build scripts to produce zip packages for `win-arm` and `win-arm64` (#5664)
- Enable `Install-Debian` to work with VSTS Hosted Linux Preview (#5659)
- Add `linux-arm` tarball package to release build (#5652, #5660)
- Enable building for `win-arm` and `win-arm64` (#5524)
- Make macOS package require 10.12 or newer (#5649, #5654)
- Update signing subjects to something meaningful (#5650)
- Make `New-UnixPackage` more readable (#5625)
- Update `PowerShellGet` tests to validate the new install location of `AllUsers` scope. (#5633)
- Increase reliability of flaky test that fails intermittently in CI (#5641)
- Exclude markdown files from `Pester` folder from the Markdown meta test (#5636)
- Run tests for Windows installer only on Windows (#5619)
- Suppress the expected errors from `Select-Xml` tests (#5591)
- Add retry logic to prerequisite URL and output URL on failure so you can more easily troubleshoot (#5601, #5570)
- Make sure submodule are initialized when running Mac release build (#5496)
- Remove duplicate files in Windows packages in a folder called `signed` (#5527)
- Add PowerShell VSCode style settings (#5529) (Thanks @bergmeister)
- Add Travis CI matrix for improved job tagging (#5547)
- Remove community docker files from official docker image validation (#5508)

* Documentation and Help Content

- XML documentation fix for `CompletionResult` (#5550) (Thanks @bergmeister!)
- Change synopsis of `install-powershell.ps1` to reflect that it works cross-platform (#5465) (Thanks @bergmeister!)
- Add more helpful message for `AmbiguousParameterSet` exception (#5537) (Thanks @kvprasoon!)
- Update the contribution guideline to note that updating the changelog is required. (#5586)
- Updated doc to build arm/arm64 versions of `psrp.windows` and `PowerShell.Core.Instrumentation.dll` libraries (#5668)
- Update Contribution guidelines with work in progress guidance (#5655)
- Update code coverage tests to get GitCommitId using the ProductVersion from Assembly (#5651)
- Remove requirement to updating changelog update in PR (#5644, #5586)
- Minor refactoring of the release build scripts (#5632)
- Update PowerShell executable name in `using-vscode.md` (#5593)
- Fix xUnit test for PS (#4780)
- Update install link and instructions for R-Pi (#5495)

* Compliance Work

Compliance work is required for Microsoft to continue to sign and release packages from the project as official Microsoft packages.

- Remove `PerformWSManPluginReportCompletion`, which was not used, from `pwrshplugin.dll` (#5498) (Thanks @bergmeister!)
- Remove exclusion for hang and add context exception for remaining instances (#5595)
- Replace `strlen` with `strnlen` in native code (#5510)
