pekko-projection-bom is not published for Scala 3

[pjfanning]

• just publishes for Scala 2.12/2.13
• might be easier to sort out after https://github.com/apache/pekko-projection/issues/110 is fixed
• see https://repository.apache.org/content/groups/snapshots/org/apache/pekko/

[pjfanning]

this is working now