Check for and eliminate any drift in the thanos chart

Over time drift has been noticed in some charts and we would like to minimize it.

Expected outcome:

• Pull down a copy of the upstream chart at the same version as ours and diff them.
  • Move any missing changes over and/or update the DEVELOPMENT_MAINTENANCE.md documentation to specify changes that are intentionally omitted (if they are not already specified).
• Check the values.yaml file
  • Remove unused/orphaned entries in the values.yaml file. Be certain they are unused before removing them.
  • If there are required values.yaml modifications that are not documented in DEVELOPMENT_MAINTENANCE.md, please update the doc accordingly.
• Increment our chart's -bb.X version
• Add a CHANGELOG.md entry.
• Regenerate README.md.