[gdb/testsuite] Improve untested message in gdb.ada/finish-var-size.exp

[gdb/testsuite] Improve untested message in gdb.ada/finish-var-size.exp

I came across:
...
UNTESTED: gdb.ada/finish-var-size.exp: GCC too told for this test
...
The message only tells us that the compiler version too old, not what compiler
version is required.

Fix this by rewriting using required:
...
UNSUPPORTED: gdb.ada/finish-var-size.exp: require failed: \
  expr [gcc_major_version] >= 12
...

Tested on x86_64-linux.