…gnostic (#8586)

…bject preventing GC from triggering finalizeReferent of phantom reference) (#8614)

Backports #8617 + fixed two links which are going to be fixed in #8616 for the master branch already.

Backport #8601.

[2.6.x] Akka 2.5 only for snapshot builds

The JavaTesting page does not have any hyperlinks to the above 2 pages, so if we would like the ScalaTest and specs2 libraries to be used by Java Developers, it may be useful to have links to them.

(manual cherry pick of 4b6b1c3)

Backports #8385.

#8648 didn't backport cleanly.

There were 2 types of problems:

1. edits on missing files (migration27.md, etc...)
2. CONFLICTs on `javaForms` wrt validation versions (JSR-303 or JSR-380). I preferred the wording from the `2.6.x` branch which still uses `hibernate-validator % 5.4.1.Final`.

## Fixes

`AkkaHttpServer` didn't honour some HTTPS-related settings:

````
play {
  server {
    # HTTPS configuration
    https {
      # Whether JSSE want client auth mode should be used. This means, the server
      # will request a client certificate, but won't fail if one isn't provided.
      wantClientAuth = false

      # Whether JSSE need client auth mode should be used. This means, the server
      # will request a client certificate, and will fail and terminate the session
      # if one isn't provided.
      needClientAuth = false
    }
  }
}
```

* Lookup form helpers' _label and _name attributes in messages

* Add test