Revise merge request pipeline job triggering rules

Revise merge request pipeline job triggering rules

Over the past few years, some of the initial decisions made about which
GitLab CI jobs to run for all merge requests and which of them to run
just for scheduled/web-triggered pipelines turned out to be less than
ideal in practice: test coverage was found to be too lax in some areas
and on the other hand unnecessarily repetitive in others.  For example,
compilation failures for certain build types that are not exercised for
every merge request (e.g. FIPS-enabled builds) turned out to be much
more common in practice than e.g. test failures happening only on a
subset of releases of a given Linux distribution.

To limit excessive resource use while retaining broad test coverage,
adjust GitLab CI job triggering rules for merge request pipelines as
follows:

  - run all possible build jobs for every merge request; compilation
    failures triggered for build flavors that were only tested in
    scheduled pipelines turned out to be surprisingly commonplace and
    became a nuisance over time, particularly given that the run times
    of build jobs are much lower than those of test jobs,

  - for every merge request, run at least one system & unit test job for
    each build flavor (e.g. sanitizer-enabled, FIPS-enabled,
    out-of-tree, tarball-based, etc.),

  - limit the amount of test jobs run for each distinct operating
    system; for example, only run system & unit test jobs for Ubuntu
    24.04 Noble Numbat in merge request pipelines, skipping those for
    Ubuntu 22.04 Jammy Jellyfish and Ubuntu 20.04 Focal Fossa (while
    still running them in other pipeline types, e.g. in scheduled
    pipelines),

  - ensure every merge request is tested on Oracle Linux 8, which is the
    operating system with the oldest package versions out of the systems
    that are still supported by this BIND 9 branch,

  - decrease the number of test jobs run with sanitizers enabled while
    still testing with both ASAN and TSAN and both GCC and Clang for
    every merge request.

These changes do not affect the set of jobs created for any other
pipeline type (triggered by a schedule, by a GitLab API call, by the web
interface, etc.); only merge request pipelines are affected.