ci: repurpose "linux-gcc" job for deprecations

ci: repurpose "linux-gcc" job for deprecations

The "linux-gcc" job isn't all that interesting by itself and can be
considered more or less the "standard" job: it is running with a
reasonably up-to-date image and uses GCC as a compiler, both of which we
already cover in other jobs.

There is one exception though: we change the default branch to be "main"
instead of "master", so it is forging ahead a bit into the future to
make sure that this change does not cause havoc. So let's expand on this
a bit and also add the new "WITH_BREAKING_CHANGES" flag to the mix.

Rename the job to "linux-breaking-changes" accordingly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>