Drop support for Db2

[lmsurpre]

Is your feature request related to a problem? Please describe.
Given the cost/benefit of supporting a unique multi-tenant schema design for Db2, I think it would be in our best interest to drop this support (so we can focus elsewhere).

Describe the solution you'd like
Unless other users/contributors step forward in support of Db2, we should remove it wholesale.
This includes at least:

• removing the db2 driver (jcc) from the pom.xml and the built installer/container images
• datasource server.xml and configDropins related to Db2
• removing db2 from the CI pipeline (replace it with postgresql and/or use chance to conslidate the overall number of tests)
• any db2-specific code in fhir-database-util, fhir-persistence-schema, and fhir-persistence jdbc
  • anything mentioning "multi-tenant schema" could also be removed
• project documentation

Describe alternatives you've considered
Continue to support db2, but drop the multi-tenant schema part of it (so it works like all our other db types).

Acceptance Criteria

1. GIVEN [a precondition]
   AND [another precondition]
   WHEN [test step]
   AND [test step]
   THEN [verification step]
   AND [verification step]

Additional context
Please contact us or comment here if you are dependent on Db2 to continue being supported out-of-the box.

[lmsurpre]

linking to a review comment that hints that we may be able to remove some related classes when we remove Db2:
#3754 (comment)