chore: remove leftover Scala 2.12 idioms

[pjfanning]

Motivation

The codebase still carried the Scala 2.12-era immutable.Seq qualifier (and its scala.collection.immutable / scala.collection.immutable.Seq imports). On Scala 2.13 and 3, scala.Seq is already an alias for scala.collection.immutable.Seq, so the qualifier is redundant. This is the pekko-management equivalent of apache/pekko#3539.

Unlike the core repo, there were no other 2.12 idioms to clean up here: no WrappedArray, filterKeys/mapValues, .toIterator, Stream, or Either projections, and no paradox anchors referencing immutable.Seq.

Modification

• Replace immutable.Seq with Seq across 24 files and drop the now-unused scala.collection.immutable, scala.collection.immutable.Seq and scala.collection.{ immutable => im } imports.
• Files that still use other immutable.* types (Set, Iterable, SortedSet) keep the scala.collection.immutable import (ClusterHttpManagementProtocol, BootstrapCoordinator, PodDeletionCostAnnotator).
• ClusterHttpManagementRoutesSpec: swap the scala.collection.immutable._ wildcard for an explicit SortedSet import.
• ConsulServiceDiscovery: drop the redundant scala.collection.immutable.Seq(targets: _*) copy, since targets is already an immutable Seq.

Result

No remaining Scala 2.12 collection idioms in main or test sources. Source- and binary-compatible: Seq has the same erasure, so MiMa passes without new filters.

Tests

• native scalafmt --mode diff-ref=upstream/main
• sbt +Test/compile passes on Scala 2.13, 3.3 and 3-next
• sbt <module>/mimaReportBinaryIssues passes for all touched modules (management, management-cluster-http, management-cluster-bootstrap, discovery-kubernetes-api, discovery-marathon-api, discovery-aws-api, discovery-aws-api-async, discovery-consul, lease-kubernetes, rolling-update-kubernetes)
• sbt <module>/test passes for all touched modules; ConsulDiscoverySpec not run locally as it requires Docker/testcontainers (relying on CI)

References

None - equivalent of apache/pekko#3539