Remove redundant Scala 2.12-era immutable.Seq imports

[pjfanning]

Motivation

Since Scala 2.13, scala.Seq is a type alias for scala.collection.immutable.Seq, and Set, Vector, Nil and friends in the scala package already refer to the immutable collections. The scala.collection.immutable imports and immutable.Seq qualifiers carried over from the Scala 2.12 days are therefore redundant now that this project only builds on 2.13 and 3.

This is the pekko-persistence-jdbc counterpart of apache/pekko#3539. Of the idioms cleaned up there, only the immutable.Seq one exists in this repo — there are no WrappedArray, filterKeys/mapValues, Either projection, Stream or .toIterator uses to touch.

Modification

• Drop the now-unused scala.collection.immutable imports (immutable._, immutable.Seq, immutable.Set, immutable.{ Nil, Seq }, immutable.{ Seq, Vector }) across 23 files.
• Replace immutable.Seq / scala.collection.immutable.Seq with plain Seq.
• MissingElements keeps its immutable.NumericRange import since NumericRange is not aliased in scala._.

scala.Seq erases to the same class, so the binary shape is unchanged. The one place where dropping immutable._ changes a resolved type is a local def next(id: String): Iterable[String] inside JdbcReadJournal.persistenceIds (now collection.Iterable instead of immutable.Iterable), which is a closure-local helper passed to statefulMapConcat and not part of any API.

Result

No remaining Scala 2.12 collection idioms in main or test sources.

Tests

• native scalafmt --mode diff-ref=upstream/main
• sbt "+core/Test/compile" on Scala 2.13.18, 3.3.8 and 3.9.0
• sbt core/mimaReportBinaryIssues passes (with mimaReportSignatureProblems := true)
• sbt "core/Test/testOnly ...TrySeqTest ...BaseDaoSpec ...H2LimitWindowingStreamTest ...JdbcAsyncWriteJournalShutdownTest ...H2ScalaCurrentPersistenceIdsTest ...ByteArrayJournalSerializerTest": 16 tests, all passed against H2
• git diff --check clean
• No new directional test: this is a pure import/alias cleanup with no behavior change; the existing specs covering the touched code were run above.

References

Refs apache/pekko#3539