AJ-1784: scalaVersion to 2.13.15; update docker images to match

[davidangb]

supersedes #1440 .

This PR:

• updates the Scala version from 2.13.14 to 2.13.15
• updates the scala-sbt docker image in use by builds etc to the correct version: Java 17, sbt 1.10.2, Scala 2.13.15
• updates the sbt-scoverage plugin to the version compatible with Scala 2.13.15

Have you read CONTRIBUTING.md lately? If not, do that first.

I, the developer opening this PR, do solemnly pinky swear that:

• I've followed the instructions if I've made any changes to the API, especially if they're breaking changes
• I've updated the FISMA documentation if I've made any security-related changes, including auth, encryption, or auditing

In all cases:

• Get two thumbsworth of review and PO signoff if necessary
• Verify all tests go green
• Squash and merge. Make sure your branch deletes; GitHub should do this for you.
• Test this change deployed correctly and works on dev environment after deployment