[3.0.x] switch away from deprecated UpgradeToWebSocket (backport #13763) by @pjfanning

[mergify]

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• Have you read through the contributor guidelines?
• Have you referenced any issues you're fixing using commit message keywords?
• Have you added copyright headers to new files?
• Have you checked that both Scala and Java APIs are updated?
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• Have you added tests for any changed functionality?

Helpful things

Fixes

Fixes #13744

Purpose

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Background Context

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References

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---

This is an automatic backport of pull request #13763 done by [Mergify](https://mergify.com).

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-04-22 11:12 UTC · Rule: default
• ✅ Checks skipped · PR is already up-to-date
• ✅ Merged — 2026-04-22 11:15 UTC · at f0066d2da02c7cb834a71b6ac53fce6b5ddf50ea · merge

This pull request spent 3 minutes 8 seconds in the queue, including 1 second running CI.

Required conditions to merge

• #approved-reviews-by >= 1 [🛡 GitHub branch protection]
  • [3.0.x] switch away from deprecated UpgradeToWebSocket (backport #13763) by @pjfanning #13894
• #changes-requested-reviews-by = 0 [🛡 GitHub branch protection]
  • [3.0.x] switch away from deprecated UpgradeToWebSocket (backport #13763) by @pjfanning #13894
• any of [🛡 GitHub branch protection]:
  • check-success = Finish / Ready To Merge
  • check-neutral = Finish / Ready To Merge
  • check-skipped = Finish / Ready To Merge

[mkurz]

@Mergifyio backport 2.9.x

[mergify]

backport 2.9.x

✅ Backports have been created

Details

• #13938 [2.9.x] [3.0.x] switch away from deprecated UpgradeToWebSocket (backport #13763) by @pjfanning (backport #13894) by @mergify[bot] has been created for branch 2.9.x but encountered conflicts

Cherry-pick of f0066d2 has failed:

On branch mergify/bp/2.9.x/pr-13894
Your branch is up to date with 'origin/2.9.x'.

You are currently cherry-picking commit f0066d2da.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   transport/server/play-akka-http-server/src/main/scala/akka/http/play/WebSocketHandler.scala
	both modified:   transport/server/play-akka-http-server/src/main/scala/play/core/server/AkkaHttpServer.scala

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally