Rename asdf-exec to tool-version-manager-exec

What does this merge request do and why?

Rename asdf-exec to tool-version-manager-exec

• Find/replace instances throughout codebase

• Delete asdf-available shell script

• Remove unused environment variables from Makefile

• mise-available functionality was copied to tool-version-manager-exec

  • A follow up MR will be made to refactor bundle-install to use tool-version-manager-exec, removing the need for mise-available entirely.

See: &19027 (closed)

How to set up and validate locally

Fresh install: curl "https://gitlab.com/gitlab-org/gitlab-development-kit/-/raw/dbarrett%2Fupdate-support-scripts-remove-asdf/support/install" | bash -s gdk "dbarrett/update-support-scripts-remove-asdf" false installs and runs successfully

Merge request checklist

• This MR references an issue describing the change.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.