Remove asdf.opt_out config setting and dependent code

What does this merge request do and why?

Remove asdf.opt_out config setting and dependent code

• Delete asdf from configuration.
• Remove from gdk where necessary and fix relevant tests.
• Delete any user notification regarding use of asdf and migration to mise.
• Update documentation accordingly.

Some things to note:

• Files in lib/asdf were left intact due to dependencies from postgres_updater.
• There are a number of areas where mise did not replace asdf, these have been left intact as it is out of scope of the migration.
• A significant number of tests were deleted that were asdf specific, where there were no mise counterparts. It is likely this will decrease coverage but follow up MRs will fix this.
• Some parts of the code, asdf_opt_out has been hardcoded to true. This is done to break up this work into smaller MRs and will be removed in follow up MRs.

See &19027 (closed)

How to set up and validate locally

We want to validate that users still on asdf won't be impacted - they should still be able to use asdf as a self-supported tool version manager.

To do this, we will install tools via asdf, check gdk installation is working then checkout this MR to ensure it is still working.

1. Install asdf: brew install asdf or one of the other methods.
2. gdk update to get the latest version of gdk on main.
3. gdk config set asdf.opt_out false && gdk config set mise.enabled false
4. gdk update to install tools via asdf - skip mise migration (N).
5. Confirm gdk is still functional using asdf installed tools.
6. Checkout this branch. git checkout remove-asdf && GDK_SELF_UPDATE=0 gdk update

There should be no mention of asdf to mise migration and gdk should still work as expected.

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.