Fix "MMM.yy" date parsing in German culture.

[pra2892]

Fix DateTime.TryParseExact for "MMM.yy" format in German culture

This commit fixes an issue where DateTime.TryParseExact fails to parse dates formatted with "MMM.yy" in German culture (de-DE). The problem occurs because the parsing logic incorrectly tries to match against genitive month names when parsing abbreviated month names without day specifiers.

According to the documentation, genitive month names should only be used when the format pattern contains day specifiers ('d' or 'dd'). The "MMM.yy" format doesn't contain day specifiers, so regular month names should be used.

Modified the MatchAbbreviatedMonthName method to conditionally use genitive month names only when appropriate, fixing scenarios like:

// s = "Dez.25"
DateTime.TryParseExact(s, "MMM.yy", new CultureInfo("de-DE"), DateTimeStyles.None, out var dt);
// Now returns true instead of false

Fixes #114159

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[pra2892]

@dotnet-policy-service agree

@dotnet-policy-service agree

[tarekgh]

@pra2892 thanks for submitting the PR. I am seeing you added submodule called runtime to this PR. Please delete it.

[tarekgh]

useGenitiveForm

I am not sure I understand this fix. the value of useGenitiveForm is always false so the condition will always be evaluated as false.

The fix should be, passing the format string to this method MatchAbbreviatedMonthName. and when checking
dtfi.FormatFlags & DateTimeFormatFlags.UseGenitiveMonth) != 0, can check if format string has d or dd specifiers.