Revert "crostini: Deprecate buster containers"

This reverts commit b7f51fa3a5b9274cb7a995436fb636f8777e0268.

Reason for revert: gardener suspect for multiple crostini.*.buster_stable failures on chromeos-eve-chrome "failed to install Crostini from UI: installation doesn't finish in time"

Original change's description:
> crostini: Deprecate buster containers
>
> Show the upgrade popup for buster containers. Testing and bug fixes for
> buster will end in the near future. Users are encouraged to upgrade to
> bullseye.
>
> Auto-formatter changes contained.
>
> Bug: b:283027529
> Test: deploy_chrome to DUT
> Change-Id: Ieef4099d28773b8259981847755174d74a6e70f6
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4680343
> Reviewed-by: David Munro <davidmunro@google.com>
> Commit-Queue: James Ye <jamesye@google.com>
> Cr-Commit-Position: refs/heads/main@{#1172216}

Bug: b:283027529, b:292012155
Change-Id: Idc799be0d632f8a3939aaacc9189b2baa11fadce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4702725
Owners-Override: Trent Apted <tapted@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: David Munro <davidmunro@google.com>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1172765}