ci: Skip flaky tests automatically and allow to opt-out

What does this MR do and why?

This makes the automatic skipping of flaky specs the default, which can be disabled with SKIP_FLAKY_TESTS_AUTOMATICALLY = "false".

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

After merge

• Remove the SKIP_FLAKY_TESTS_AUTOMATICALLY variable from https://gitlab.com/gitlab-org/gitlab/-/settings/ci_cd
• Remove the SKIP_FLAKY_TESTS_AUTOMATICALLY variable from https://gitlab.com/gitlab-org/security/gitlab/-/settings/ci_cd