ci(workflows): parameterize MySQL and PostgreSQL versions

[Tony133]

Proposal:

• Adds optional version inputs to the MySQL and PostgreSQL plugin CI workflows.

  • MySQL defaults to version 8.0
  • PostgreSQL defaults to version 18
  • The database version can now be overridden by plugin workflows
  • Maintains current behavior for existing consumers
  • PostgreSQL versions older than 17 are not supported

• This allows individual plugins to test additional database versions, if needed, following the same pattern already used by the Elasticsearch CI workflow.

• We currently support Elasticsearch versions 8 and 9 in the Elasticsearch plugin for backward compatibility.

Note:

• This prepares shared workflows for testing new major database versions without changing the default version for existing consumers.
• Individual plugins can explicitly test a newer database version when needed.