fix: ci: Increase GIT_DEPTH for the "assign-milestones" job

Cloning tags with the default GIT_DEPTH of 1 prevents the milestone
assignment script from identifying any merge requests that are included
in a given release.  Fix by increasing GIT_DEPTH to an arbitrary value
that is high enough for practical purposes.

The GIT_DEPTH CI variable defaults to 1 for all jobs through the
top-level "variables" key.  Explicitly setting it to 1 in job
definitions is unnecessary and may cause confusion.  Remove these
redundant assignments.

Merge branch 'michal/fix-assign-milestones-job' into 'main'

See merge request isc-projects/bind9!11996