Build should set PRERELEASE=0 in release/* branches

[GrabYourPitchforks]

Related: dotnet/coreclr#23436

The release/* branches (even for previews) should set PRERELEASE=0 so that we're not building anything in there that we don't intend to ship. There are still some features in master that aren't yet ready for prime time and are conditioned on the flag PRERELEASE=1 being set.

runtime/eng/native/configureplatform.cmake

Lines 4 to 6 in 4d3b5e5

	# If set, indicates that this is not an officially supported release
	# Keep in sync with IsPrerelease in Directory.Build.props
	set(PRERELEASE 1)

runtime/Directory.Build.props

Lines 63 to 65 in a1af15d

	<!-- Indicates this is not an officially supported release. Release branches should set this to false. -->
	<!-- Keep it in sync with PRERELEASE in eng/native/configureplatform.cmake -->
	<IsPrerelease>true</IsPrerelease>

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[ericstj]

@GrabYourPitchforks this means those features would never be in packages shipped to nuget.org. Is that your intent?

@Anipik let's clarify what to do here and see if we can make it happen in Preview5.

[jkotas]

this means those features would never be in packages shipped to nuget.org

Yes, that's the intent.