tune CODEOWNERS file

[c-dilks]

• simplify the CODEOWNERS file more; each module should only have the "primary" authors responsible for reviewing code
  • fix: reduce the number of CODEOWNERS #764
• pom.xml files should be owned by a few, otherwise all owners will be review-requested, e.g., on a version bump
  • fix: specify code owner of all pom.xml files #750
  • fix: pom.xml code owner must be specified last #753
• convert GitHub usernames to GitLab usernames
  • holding off on this for now, until after we move to GitLab
  • NOTE: CODEOWNERS does not work on the current Tier of our code.jlab.org GitLab instance