Modify BB MR creation to modify `oci.tag`

Currently package MR merges result in creation of a Big Bang MR: https://repo1.dso.mil/big-bang/pipeline-templates/pipeline-templates/-/blob/5bc5d1e367d337c394a7cd0f14c84f841ba4d9cf/library/templates.sh#L1289

This functionality currently targets the git block under a given package. In light of the move to OCI we should have this target the oci block if it exists, so that the OCI tag reference gets updated.

This may have a modified goal based on #124 (closed) - we may want to target both branches with different MRs (1 targeting master to update git tag, 1 targeting release-2.x to update OCI tag).