Mk/bsd.default-versions.mk: Switch default GCC from 10 to 11

Update GCC_DEFAULT, the default version of GCC as pulled in via
USE_GCC=yes and a myriad of other ways from GCC 10 (10.3 currently)
to GCC 11 (11.3 currently).

PR: 258378
Tested by: antoine (exp-runs)
Thanks to: salvadore (for heavy lifting across the board)