Update Scala 2.13 to 2.13.15

[piotrp]

Describe your changes

We build Nu with Scala 2.13.12 and use Tapir built with Scala 2.13.13, which is incompatible - see SIP-51 for details. If we were using sbt 1.10.0+ our build would fail.

Checklist before merge

• [ ] Related issue ID is placed at the beginning of PR title in [brackets] (can be GH issue or Nu Jira issue)
• [x] Code is cleaned from temporary changes and commented out lines
• [ ] Parts of the code that are not easy to understand are documented in the code
• [ ] Changes are covered by automated tests
• [ ] Showcase in dev-application.conf added to demonstrate the feature
• [ ] Documentation added or updated
• [ ] Added entry in Changelog.md describing the change from the perspective of a public distribution user
• [ ] Added MigrationGuide.md entry in the appropriate subcategory if introducing a breaking change
• [ ] Verify that PR will be squashed during merge

[piotrp]

Oops, forgot about flink-scala-2.13. Publishing is in progress, I will update this PR when Maven Central will start to see the new version.

[piotrp]

Done, now with updated flink-scala-2.13