…ir resolvers

…default

Pekko HTTP 2 omits generated Content-Length headers for HEAD
responses after apache/pekko-http#962, ported from
akka/akka-http#4214. Update the shared HEAD integration spec to
expect that behavior.

Make the Netty backend match by marking HEAD responses internally and
using a Play-specific response encoder. The marker lets the Netty
reactive-streams handler keep the connection alive without exposing
Content-Length or Transfer-Encoding on the wire.

Document that HEAD responses no longer include generated
Content-Length headers, following the Pekko HTTP 2 behavior change.

Also document that clustered Pekko applications using Remote/Artery TCP
may need the jdk.internal.misc add-opens flag because of Agrona.

Document the Play upgrade to Pekko 2 and Pekko HTTP 2, including the
removal of the deprecated UpgradeToWebSocket-based low-level WebSocket
bridge overloads.

Update current manual Pekko links to the Pekko 2 documentation and API
references.

Update Scaladoc, Javadoc, and API doc generation links from Pekko 1.0
to Pekko 2.0 so generated Play docs point at the matching upstream
Pekko documentation.

Update the manual examples for overriding Pekko and Pekko HTTP
dependencies so they use Pekko 2 version numbers instead of the old
Pekko 1.x versions.