This commit replaces "Not subscribed" tab in stream settings
with "Available" tab where only streams which the user can
subscribe to are shown.
Fixes#35919.
We have not yet removed the help files. We can use this commit as a
conversion commit since a lot more help changes will be merged before
this PR gets merged. We will need to rebase on top of main, run
conversion step in this commit, push those changes and then merge the
PR.
NOTE: This commit temporarily breaks the astro build since we have
renamed the path in the MDX files from starlight_help/ to help/ early.
That should be fine since astro build will not break any tests right
now.
We should squash this commit with the next commit removing help center
files when merging into main and remove this NOTE.
