ci: merge linux-gcc-default into linux-gcc

The "linux-gcc-default" job is mostly doing the same as the "linux-gcc" job, except for a couple of minor differences: - We use an explicit GCC version instead of the default version provided by the distribution. We have other jobs that test with "gcc-8", making this distinction pointless. - We don't set up the Python version explicitly, and instead use the default Python version. Python 2 has been end-of-life for quite a while now though, making this distinction less interesting. - We set up the default branch name to be "main" in "linux-gcc". We have other testcases that don't and also some that explicitly use "master". - We use "ubuntu:20.04" in one job and "ubuntu:latest" in another. We already have a couple other jobs testing these respectively. So overall, the job does not add much to our test coverage. Drop the "linux-gcc-default" job and adapt "linux-gcc" to start using the default GCC compiler, effectively merging those two jobs into one. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>