Clarify selection process of new board members

The most important part of this MR is to clarify that we select board members so that the size of the board at the end of April 1 is no more than 14.

This changes our board selection process and thus requires a 2/3 vote. As this is an MR, however, we follow the rules for an MR vote (at the end of https://gitlab.haskell.org/hf/meta/-/blob/main/board.md), meaning that anyone who does not participate in the discussion, at all, abstains, meaning quorum is guaranteed.

The deadline for this vote is in two weeks, May 2.