Developer tooling: Migrate from asdf to mise

What does this MR do?

Developer tooling: Migrate from asdf to mise

Most GitLab projects now use mise instead of asdf to manage developer tools. This updates our developer docs and .tool-versions to a mise.toml. Mise was already compatible with the projects .tool-versions so this is mostly a documentation clean up.

This follows the recent switch to mise in Operator, but most other GitLab-project migrated from asdf to mise (formerly rtx) years ago.

Related issues

Author checklist

For general guidance, please follow our Contributing guide.

Required

For anything in this list which will not be completed, please provide a reason in the MR discussion.

• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened.

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab chart.