chore: remove leftover Scala 2.12 idioms

[pjfanning]

Motivation

The codebase still carried the Scala 2.12-era immutable.Seq qualifier throughout main, test and doc sources. scala.Seq has been an alias for scala.collection.immutable.Seq since Scala 2.13, so the qualifier and the scala.collection.immutable imports that exist only to support it are redundant. This is the pekko-projection equivalent of apache/pekko#3539.

Modification

• Replace immutable.Seq with Seq across 37 Scala files and drop the scala.collection.immutable imports that are no longer referenced (kept where immutable.IndexedSeq / immutable.Set are still used).
• Drop the explicit import scala.collection.immutable.Seq lines.
• Paradox prose (cassandra, jdbc, r2dbc, slick) updated from immutable.Seq to Seq to match the doc snippets.

The other idioms addressed upstream (WrappedArray, filterKeys/mapValues, Either projections, toIterator, Stream) do not occur in this repository, so this PR is smaller in scope than the pekko one.

Result

No remaining Scala 2.12 collection idioms in main, test or doc sources. Source- and binary-compatible: same type, same erasure, no MiMa filters needed.

Tests

• native scalafmt --mode diff-ref=upstream/main run on changed files
• sbt Test/compile jdbc-int-test/Test/compile r2dbc-int-test/Test/compile slick-int-test/Test/compile passes on Scala 2.13 and 3.3
• sbt core/mimaReportBinaryIssues jdbc/mimaReportBinaryIssues slick/mimaReportBinaryIssues cassandra/mimaReportBinaryIssues eventsourced/mimaReportBinaryIssues durable-state/mimaReportBinaryIssues r2dbc/mimaReportBinaryIssues passes (grpc has MiMa disabled)
• git diff --check clean
• Test runs: relying on CI (no behavior change)

References

None - mirrors apache/pekko#3539; Scala 2.12 support was dropped previously