Mk/bsd.default-versions.mk: Add version 11 to GCC_DEFAULT

Now that GCC 11.1 has been released and lang/gcc11 is in place,
explicitly list version 11 as an option for GCC_DEFAULT.

Technically USE_GCC=11 was already working, using lang/gcc11-devel,
alas intentionally undocumented (and definitely unsupported).

Add an explicit note that versions of GCC not listed here are not
supposed to be used. [1]

Suggested by: mandree [1]