refactor: replace deprecated collection aliases in internal io code

[pjfanning]

Motivation

scala.collection.GenTraversableOnce and scala.collection.immutable.Traversable are deprecated aliases retained for the 2.12 migration — they resolve to IterableOnce and immutable.Iterable. Three internal sites still use them, and each carries a @nowarn("msg=deprecated") to silence the resulting warning:

site	alias
DnsMessage.scala:170 — Message.parse's flattener implicit	GenTraversableOnce
TcpConnection.scala:226 — completeConnect	immutable.Traversable
TcpIncomingConnection.scala:33 — constructor	immutable.Traversable

Modification

Use the current names and drop the three @nowarn suppressions.

The suppressions are the substantive part of this change. @nowarn("msg=deprecated") on a class or method silences every deprecation warning in its scope, not just the one it was added for. I verified all three were stale by removing them and recompiling — the code builds clean without them. Left in place they would keep hiding unrelated deprecation warnings in those files indefinitely; in TcpIncomingConnection the annotation sits on the whole class.

Scope and safety

All three sites are internal:

• Message is private[internal]
• TcpConnection and TcpIncomingConnection are private[io], and the only two callers of completeConnect (TcpIncomingConnection.scala:46, TcpOutgoingConnection.scala:112) are in the same package

The aliases are identical types, so this is behaviour preserving — no erasure change, no signature change visible outside io.

What is deliberately not in this PR

The public API still uses immutable.Traversable, in Tcp.scala:138,167, Udp.scala:118,139, UdpConnected.scala:112 and stream/scaladsl/Tcp.scala:144,186,220. Those are left alone on purpose: the Scala pickle signature changes even though erasure does not, and Udp.SimpleSender is a case class so copy and unapply would change with it. That is a considered API change for a major release, gated on +mimaReportBinaryIssues, not a drive-by refactor. Their @nowarns stay for the same reason.

Tests

No new tests. This is a type-alias substitution with no behaviour change, and the affected paths already have coverage — MessageSpec exercises Message.parse and therefore the flattener implicit directly.

• actor-tests/testOnly org.apache.pekko.io.dns.* — 60 tests pass
• actor-tests/testOnly org.apache.pekko.io.dns.internal.* org.apache.pekko.io.TcpConnectionSpec org.apache.pekko.io.TcpIntegrationSpec — 94 pass
• sbt actor/mimaReportBinaryIssues — clean
• sbt ++3.3.8 actor/compile — passes. Worth calling out: GenTraversableOnce and IterableOnce differ in availability between Scala 2.13 and 3, and DnsMessage.scala is a single shared source file rather than a cross-version variant, so Scala 3 was the real risk in this change.
• scalafmt run

References

Last of the items from the Java 8 / Scala 2.12 legacy sweep, following #3465 and #3466.