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>

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