The bootstrapping script doesn't work with stack-2

[judah]

The bootstrapping script, which we use to generate new versions of the descriptor and plugin protos, doesn't work with stack-2.

Problems include:

• pantry doesn't resolve relative symlinks correctly: Git dependencies fail when they have some kinds of relative symlinks commercialhaskell/stack#5004
• stack-boostrap.yaml needs to change to the new syntax for git extra-deps
• git extra-deps aren't allowed to refer to a branch name like master, as is
  currently in stack-bootstrap.yaml

For now we can work around it by using an older version of stack, e.g., 1.9.3.