Run shotgun tests on MRs

Run shotgun tests on MRs

If the shotgun tests are executed for MRs, compare it against the MR's
base rather than the previous release. Only fail the job in case the
performance drops (pass on performance improvements).

Note that start_in optimization was removed, since it isn't properly
supported with rules as of February 2025
(https://gitlab.com/gitlab-org/gitlab/-/issues/424203). Without this
optimization, container test images are likely to be re-built
unnecessarily when testing different protocols. A workaround for the
.gitlab-ci.yml exists, but the extra complexity doesn't seem justified.
The container image builds might change or be optimized in the future,
so let's just go with the build duplication for now.