- regenerate bootstrap plans
- update CI and docker images for 8.8.4 and 8.10.4

* Changes needed for GenValidate and release.py to
  accommmodate new solver dependency
* Bumps bootstrap plans to modern GHC versions
* Update `validate.sh` and `release.py` to accommodate new solver dep.
* Update `Makefile` targets

This is based on / similar to https://github.com/haskell/cabal/pull/7219

We used import qualified M in Paths_*.hs, which might be disallowed in
GHC 8.10 when -Wprepositive-qualified-module is set.

I want to use stack diffs (https://github.com/ezyang/ghstack/

) to
submit PRs to Cabal.  Stack diffs, simply by virtue of what they
are, are PRs that are targeted to branches that are not master (they
target an appropriate base that lets GitHub render only the change
in question, even if there are other changes below it.)  However,
I don't get CI jobs on those stack diffs because the current
pull_request branch selector is too restrictive.  This PR makes
it less restrictive.

This PR is submitted using ghstack to validate that this indeed
works.

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

ghstack-source-id: b135ea67848f873fc6b50b950405859587ae8d89
Pull Request resolved: https://github.com/haskell/cabal/pull/7308

8.6 was no longer included in the image

Otherwise enabling `-Wmissing-safe-haskell-mode`
would cause annoying warnings.

The are various motivation points:
- I'm not aware anyone third party running Cabal tests (maybe nixpkgs?)
- No need to do gen-extra-source-files, as Cabal-tests is not distributed
- No multiple rebuilds of Cabal-QuickCheck, Cabal-tree-diff
- Slightly smaller Cabal sdist (which is quite big still)

A script essentially runs `v2-install cabal-install:exe:cabal`,
but does some simple setup, so the result is less luckily
to be influences by environment.

Also improve bootstrap to do out-of-tree builds.
- Use http, https tricky in some environments

The folder contains also `Lexer.x`, which isn't a template as such,
but it's not far from that, conceptually. `templates/` is better
name than `boot/`