Improve the handling of tags and branches which contains a `/` when triggering CNG pipelines

What does this MR do?

1. This uses CI_COMMIT_REF_NAME for GITLAB_REF_SLUG in scripts/trigger-build in the case of a tag. This is to ensure that tags are kept as-is and not slugified. This is a prerequisite for gitlab-org/build/CNG!189 (merged).
2. This now passes GITLAB_VERSION instead of GITLAB_CE_VERSION/GITLAB_EE_VERSION to the triggered CNG pipeline because there was no reason to include the edition in the variable name.
3. This now passes GITLAB_ASSETS_TAG to the triggered CNG pipeline so that gitlab-{ce,ee} is the SSOT for this assets tag (since the assets image is built in scripts/build_assets_image.
4. This uses $CI_COMMIT_REF_SLUG in scripts/review_apps/review-apps.sh instead of $CI_COMMIT_REF_NAME so that / are replaced by -, now that the CNG pipeline will use GITLAB_REF_SLUG to tag the various CNG images (see gitlab-org/build/CNG!189 (merged)).

• This goes hand in hand with gitlab-org/build/CNG!189 (merged) which addresses the CNG side of the problem.
• This merge request should be merged first since it's back-compatible with the current CNG's .gitlab-ci.yml while gitlab-org/build/CNG!189 (merged)is merged.

What are the relevant issue numbers?

Related to https://gitlab.com/charts/gitlab/issues/1068.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer