Drop support for RHEL6 for .NET 5+

[jkotas]

https://access.redhat.com/support/policy/updates/errata/#Maintenance_Support_2_Phase has Product Retirement for RHEL6 on November 30, 2020.

[jkotas]

Confirmed over email with @richlander

[jkotas]

Working around missing features in 10+ year old Linux kernel is getting more and more painful, e.g. #420 (comment).

[wfurt]

what is next step @jkotas? Should we remove builds and test runs?

[jkotas]

Right.

[ViktorHofer]

Those are already removed so I'm closing the issue. Let me know if we should re-open in case we need to remove source code as well.

[jkotas]

Yes, it would be nice to mop-up RedHat 6 related code:

Search for "rhel6":

                if ((line.find(rhel6Prefix) == 0) || (line.find(centos6Prefix) == 0))	d:\runtime\src\installer\corehost\common\pal.unix.cpp
#   |   rhel6   |           rhel.6-x64              |	d:\runtime\src\coreclr\init-distro-rid.sh
                pal::string_t rhel6Prefix(_X("Red Hat Enterprise Linux Server release 6."));	d:\runtime\src\installer\corehost\common\pal.unix.cpp
        // RHEL6 uses a version of OpenSSL that (empirically) doesn't support designated responders.	d:\runtime\src\libraries\System.Security.Cryptography.X509Certificates\tests\RevocationTests\DynamicRevocationTests.cs
        // (There's a chance that we should be passing in extra stuff, but RHEL6 is the only platform	d:\runtime\src\libraries\System.Security.Cryptography.X509Certificates\tests\RevocationTests\DynamicRevocationTests.cs

Search for "RedHatFamily6":

        private static readonly bool s_supportsDesignatedResponder = PlatformDetection.IsNotRedHatFamily6;	d:\runtime\src\libraries\System.Security.Cryptography.X509Certificates\tests\RevocationTests\DynamicRevocationTests.cs
                   return PlatformDetection.IsNotRedHatFamily6 && (NativeLibrary.TryLoad("libgdiplus.so", out _) || NativeLibrary.TryLoad("libgdiplus.so.0", out _));	d:\runtime\src\libraries\Common\tests\CoreFx.Private.TestUtilities\System\PlatformDetection.cs
        public static bool IsRedHatFamily6 => IsRedHatFamilyAndVersion(6);	d:\runtime\src\libraries\Common\tests\CoreFx.Private.TestUtilities\System\PlatformDetection.Unix.cs
        public static bool IsNotRedHatFamily6 => !IsRedHatFamily6;	d:\runtime\src\libraries\Common\tests\CoreFx.Private.TestUtilities\System\PlatformDetection.Unix.cs
        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotRedHatFamily6))]	d:\runtime\src\libraries\System.Diagnostics.Process\tests\ProcessTests.Unix.cs
        public static bool GetIsDrawingSupported() => /* ActiveIssue(24525) */ PlatformDetection.IsNotRedHatFamily6 && PlatformDetection.IsDrawingSupported;	d:\runtime\src\libraries\System.Drawing.Primitives\tests\Helpers.cs
        static bool IsBindMountSupported => RuntimeInformation.IsOSPlatform(OSPlatform.Linux) && !PlatformDetection.IsInContainer && !PlatformDetection.IsRedHatFamily6;	d:\runtime\src\libraries\System.IO.FileSystem\tests\Directory\Delete.cs
        static bool IsBindMountSupported => RuntimeInformation.IsOSPlatform(OSPlatform.Linux) && !PlatformDetection.IsInContainer && !PlatformDetection.IsRedHatFamily6;	d:\runtime\src\libraries\System.IO.FileSystem\tests\File\Delete.cs
        [ConditionalFact(typeof(PlatformDetection), "IsNotRedHatFamily6")]	d:\runtime\src\libraries\System.IO.Pipes\tests\AnonymousPipeTests\AnonymousPipeTest.Specific.cs
        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotRedHatFamily6))]	d:\runtime\src\libraries\System.Runtime.Extensions\tests\System\Runtime\ProfileOptimization.cs