[17u]: GHA update: Use xcode 12.4. for macos-x64

[Lindenmaier, Goetz]

Hi,

I would like to propose to update the compiler
used in GHA for macos-x64 from xcode 11.7 to xcode 12.4.

We already build macos-aarch64 with this compiler.
jdk21u uses xcode 12.5.1 in the GHA.
At SAP, we build with xcode 13 for jdk17, so this is tested already.

I think it is beneficial to have the same compiler for both
macos builds, and to be closer to jdk21.
Also, I assume more people are building with recent compilers
than with xcode 11 on mac.

Best regards,
  Goetz


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