Tracking: avoid suppressing SYSLIB0001 in shipping sources

There are a few cases in the sources where aspnet is comparing if (encoding == Encoding.UTF7) { /* do something */ }. Since this accesses the Encoding.UTF7 property getter, a warning is produced.

From https://github.com/dotnet/docs/issues/19274:

If you’re trying to compare an Encoding instance against Encoding.UTF7, consider instead performing a check against the well-known UTF-7 code page (65000). This has the dual advantage that it both avoids the warning and handles some edge cases (such as somebody having subclassed UTF7Encoding).

void DoSomething(Encoding enc)
{
    // don't perform the check this way; it produces a warning and misses some edge cases
    if (enc == Encoding.UTF7)
    {
        // encoding is UTF-7
    }

    // instead, perform the check this way
    if (enc != null && enc.CodePage == 65000)
    {
        // encoding is UTF-7
    }
}

[BrennanConroy]

Fixed via https://github.com/dotnet/aspnetcore/pull/28913

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