Integrate gitlab-ci/use-two-dashes-as-separator and 2 more

Commit: https://gitlab.com/garz-fricke/yocto/infrastructure/gitlab-ci/-/commit/09eb4bcfa363e2674a6986cebaf5d6eba7c8fdd7

list_commits: use "--" as separator

Unfortunately, GitLab still interpreted three of the four dashes as start and end of YAML syntax, so we go back to two dashes.

--

Commit: https://gitlab.com/garz-fricke/yocto/infrastructure/gitlab-ci/-/commit/a3cab669171774b7bfb620a51b2db50965a2592a

list_commits: use "----" as separator

The former "---" was still rendered as YAML syntax by GitLab in some cases.

--

Commit: https://gitlab.com/garz-fricke/yocto/infrastructure/gitlab-ci/-/commit/625197900307693db66987bb1987822b45c4d447

create_merge_request: optimize description

Restore the "---" separator which was removed in a previous commit. The description simply must not start with this separator. If we include it only in between the blocks, all instances are rendered as a horizontal line, which makes the MR description more readable. Include a helper function for this purpose, which extracts the message body from a commit message.