Prepare for buttons release
Core 5.31 is due out next week. The only difference from cdntax 1.5.1 to 1.5.2 is the buttons changes needed for 5.31.
For version compatibility, I've tentatively put 5.28 (last security release). While it works with 5.28+, nobody has to upgrade until they install core 5.31.
In any case the civi directory won't signal a release until we push a tag. So we could also time that with the core release independent of this PR.