[15u notice] jdk 15.0.8 last merge on Monday, Jun 20

[Yuri Nesterenko]

Hi,

the last regular merge from jdk15u-dev to jdk15u will be on Monday, Jun 20;
jdk15u-dev will be retargeted to October release.
Please push the approved PRs now.

Since then, a -critical- tag would be required to get a change to July release
via direct push to jdk15u.

Thanks,
--yan