remove the Akka 2.6.4 rolling-migration manifests from ClusterMessageSerializer

[pjfanning]

Motivation

ClusterMessageSerializer still accepted the long class-based manifests kept "for one
version iteration from Akka 2.6.4 to allow rolling migration to short manifests", marked
removable "in Akka 2.6.6 or later" — the thirteen Old* / *Pre2523 vals and their
fromBinary branches.

No peer a Pekko node can talk to emits them:

• Pekko has always emitted the short manifests (J, W, GE, …) — the long form was
  only ever sent by Akka ≤ 2.6.4.
• An actual old Akka node would send akka.cluster.InternalClusterAction$Join and
  friends. These constants were renamed to org.apache.pekko.cluster.* by the fork's
  package rename, and nothing in pekko-remote translates manifests, so they can never
  match what such a node sends either.

So every branch is dead code — and each one is a wire-reachable deserialization path,
which is exactly the surface the recent serializer hardening has been shrinking.

Modification

Remove the vals, their fromBinary cases, and the two heartbeat deserializers only those
cases used (deserializeHeartBeatAsAddress, deserializeHeartBeatRspAsUniqueAddress).

In the spec, remove the old-manifest round-trip tests and their now-unused helpers
(roundtripWithManifest, checkDeserializationWithManifest), and add a test asserting
every removed manifest is now rejected as an unknown manifest.

Result

The class-based manifests are refused with IllegalArgumentException("Unknown manifest [...]") like any other unrecognised manifest. Handling of the short manifests is
unchanged.

Tests

• sbt "cluster/testOnly org.apache.pekko.cluster.protobuf.ClusterMessageSerializerSpec" — 8 passed
• The new rejection test was checked to discriminate by reverting the production change
  and re-running: it then fails because the old manifest routes into deserializeJoin,
  which raises InvalidProtocolBufferException on the test payload instead of the
  unknown-manifest rejection.
• sbt "cluster/mimaReportBinaryIssues" — no issues
• sbt "cluster/scalafmtCheckAll" — clean

References

None — removes dead Akka 2.6.4 compatibility code.