[release/6.0] Use simpler Docker tags (#45117)

- `cherry-pick` of 5f4bc82d, Update docker tag to latest schema
  - This change moves all of the docker images to the -latest tag as part of #10377.
- `cherry-pick` of 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.
- via #45115, w/ a slight adjustment from rhel-7 to centos-7 in rhel.Dockerfile

nit: Correct timeout of source-build job
- not honoured as a top-level parameter to source-build.yml

Co-authored-by: Michelle McDaniel <michelm@microsoft.com>