Users can parse this file to get the shasum they require.

Remove `nvim-linux64.tar.gz` and `nvim.appimage` as maintaining
these is too much work.

Also fix directory names to be consistent.

Problem: No releases for ARM Linux.

Solution: Provide appimages and tarballs for `linux-arm64`. Rename
x86 releases to `linux-x86_64` for consistency.

Problem: Some CI jobs are redundant: `RelWithDebInfo` is already tested
on Linux-Arm64; `MinSizeRel` and Ninja Multi Config are not sufficiently
relevant in practice to spend CI cycles on.

Solution: Remove `build-types` job.

Problem: Linux `aarch64`/`arm64` builds are not tested.

Solution: Add `ubuntu-arm` runners to test matrix (using
`RelWithDebInfo` build).

This is because we reference to deprecated.txt from news.txt, so
deprecation news updates are made only in deprecated.txt.

There is a "performance" section in news.txt so it makes sense we should
also give a reminder to update news for performance improvements.

A new security restriction in Ubuntu 24.04 prevents users from using
`unshare`, so we need to disable it in order for the test to work
properly.