Fixes: dotnet/maui#10901

Building a .NET MAUI project such as:

    dotnet new maui
    dotnet build -c Release -f net6.0-android -p:AndroidLinkMode=r8

Results in build warnings from R8 such as:

    R8 : warning : Missing class androidx.window.extensions.WindowExtensions

In 92bc705, we already had attempted to silence warnings from R8 such as:

    R8 : warning : Resource 'META-INF/MANIFEST.MF' already exists.

At the time, there was no option to hide warnings, now there is!

    --map-diagnostics[:<type>] <from-level> <to-level>
        # Map diagnostics of <type> (default any) reported as
        # <from-level> to <to-level> where <from-level> and
        # <to-level> are one of 'info', 'warning', or 'error' and the
        # optional <type> is either the simple or fully qualified
        # Java type name of a diagnostic. If <type> is unspecified,
        # all diagnostics at <from-level> will be mapped.
        # Note that fatal compiler errors cannot be mapped.

We can pass:

    --map-diagnostics warning info

Which can be done in existing apps via:

    <AndroidD8ExtraArguments>--map-diagnostics warning info</AndroidD8ExtraArguments>
    <AndroidR8ExtraArguments>--map-diagnostics warning info</AndroidR8ExtraArguments>

To solve this problem, let's create a new `$(AndroidD8IgnoreWarnings)`
MSBuild property and make use of one we already have
`$(AndroidR8IgnoreWarnings)`:

    <ItemGroup>
      <_AndroidD8MapDiagnostics Condition=" '$(AndroidD8IgnoreWarnings)' == 'true' " Include="warning" To="info" />
      <_AndroidR8MapDiagnostics Condition=" '$(AndroidR8IgnoreWarnings)' == 'true' " Include="warning" To="info" />
    </ItemGroup>

We can then use these item groups to pass `--map-diagnostics` as
appropriate. Developers can turn off either property to disable this
behavior.

We can then remove the weird `Regex` code added in 92bc705. The
existing tests from 92bc705 should also be sufficient for testing
this change.