chore(backend): update Flyway to 11.20.3

[JuliusHenke]

Dependency upgrade increment B7b. Runs on Spring Boot 2.7, deployed on its own before the Boot 3 migration because it touches the database migration tooling.

• flyway-core 8.5.13 → 11.20.3 (last 11.x, the line Boot 3.5 manages).
• Added flyway-database-postgresql 11.20.3 (Postgres support moved into this module in Flyway 10).
• No SQL migrations or configuration changes.

Tested locally on JDK 17:

• ./gradlew test passes (69 tests, 0 failures) against Postgres 18.
• Upgrade in place: a Postgres 18 database migrated by the previous image (Flyway 8.5.13) was started with the new image: "Successfully validated 3 migrations", "Schema public is up to date. No migration necessary", the app starts, and flyway_schema_history is identical (same 3 rows, checksums unchanged).

Deployment notes: take a database backup before the first start of this version. The history table was not modified in the local test, so rolling back to the previous image should work, but that was not tested.

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