Update akka to 2.6.21

[xuwei-k]

Pull Request Checklist

• Have you read How to write the perfect pull request?
• 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?
• Have you updated the documentation for both Scala and Java sections?
• Have you added tests for any changed functionality?

Helpful things

Fixes

Purpose

update akka

Background Context

References

• akka/akka@v2.6.20...v2.6.21
• https://github.com/akka/akka/releases/tag/v2.6.21

[rmmeans]

Thanks for getting this PR in! We've been hit pretty hard with this issue and had to switch our Play apps running latest 2.8 versions back to the netty backend to resolve production performance issues.

[mkurz]

Sorry, I had a bit of a family emergency the last two weeks which took all my capacity, I will try to merge and cut a new Play 2.8.x this weekend.

[mkurz]

@Mergifyio backport 2.8.x

[mergify]

backport 2.8.x

✅ Backports have been created

• #11874 [2.8.x] Update akka to 2.6.21 (backport #11849) by @xuwei-k has been created for branch 2.8.x but encountered conflicts

[mkurz]

Shipped in just released Play 2.8.20: https://github.com/playframework/playframework/releases/tag/2.8.20