Add support for aliases like airflow-2 in Composer image version field

[modular-magician]

This covers the new aliases for Airflow major version (airflow-1, airflow-2) in image version field, and updates to the related documentation.

I also switched Composer tests to use the new Composer and Airflow image version aliases. Instead of finding the latest available image version for the given Composer and Airflow major versions, the new image version aliases are
used to let Composer resolve them on the server side: composer-1-airflow-1, composer-1-airflow-2, composer-2-airflow-2.

Fixes #11390

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

composer: Added support for `airflow-1` and `airflow-2` aliases in image version argument

Derived from GoogleCloudPlatform/magic-modules#5885

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.