graphics/rawtherapee: Revert "pin GCC version to 13."

This reverts commit 7c091b1abc84f4246e557aaf7d04d111e4db9b8f.

Reason I am reverting this is that I can set USE_GCC=14 and
happily compile, without or with LTO and/or native.

My conclusion is that rawtherapee is doing its things right,
but one of its libraries gets compiled for libstdc++, and pollutes
rawtherapee's program space but we don't give rawtherapee libstdc++
but libc++ on systems where the default compiler uses the latter.

PR: 284487