[release/3.1] Use simpler Docker tags (#45118)

- analogous to 5f4bc82d, Update docker tag to latest schema
  - This change moves all of the docker images to the -latest tag as part of #10377.
- analogous to f79f2d1c, Replace -latest docker tags with new schema
  - We changed the latest tags to not include the -latest suffix, so this change updates those tags to the new schema.
- slight adjustment to images: from rhel-7-rpmpkg to centos-7-rpmpkg in rhel.Dockerfile

nit: update `trigger` branch names