[VS Code] Move from asdf to mise for dependency management

See https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/mise.md

On March 1, the GDK will officially move over from asdf to mise for dependency management.

In the GitLab Workflow project we have a .tool-versions which provides dependency instructions to asdf, pretty soon most engineers at GitLab will be using mise which reads from a mise.toml file instead. For ease of onboarding / cross-team contributions, we should converge on the common tool.

Other projects such as the AI Gateway have already made the switch.

Migration should be relatively simple, consisting of the following:

Proposal

• Before March 1: Migrate .tool-versions over to mise.toml.
• Communicate the plan to devs - they can run mise install to install local dependencies.
• On or after March 1: Remove the .tool-versions file.