Planning the next release - 2.30

If we stick to our regular schedule, the next Math/Stan/CmdStan and stanc3 releases are about five weeks away - feature freeze would happen on the 16th of May and the release ten days later.

The last release (2.29.0) happened on the 14th of February, but we also had two patch releases (on the 2nd and 25th of March). Both releases were done from develop/master branches - meaning that everything merged between the 2.29.0 release and both patch releases were only bug fixes. This would mean that the 2.30 would only include about seven weeks of development if we follow the current schedule.

Given these circumstances, I would propose moving the release a month - so that the feature freeze would happen on the 16th of June, and the actual release would again follow in about ten days.

The biggest feature of the next release is set to be complex containers (complex vector, row_vector and matrix) and corresponding functions supporting those inputs.

If anyone objects to moving the release a month, please come forward until the 25th of April. If there are no objections, we will move along with the new proposed plan.