Emacs 27.1 was released nearly 6 years ago (August 2020). Bumping
the minimum version lets us remove ~30 lines of compatibility code:

- Remove fileloop fallback using deprecated tags-loop-scan/operate
- Fix tags-query-replace FIXME in projectile-replace-regexp by using
  fileloop-initialize-replace + fileloop-continue
- Replace projectile-flatten with built-in flatten-tree (6 call sites)
- Simplify projectile-time-seconds to just (time-convert nil 'integer)
- Use proper-list-p for known-projects file validation
- Use json-parse-buffer unconditionally in CMake preset reader
- Use package-get-version unconditionally
- Add (require 'fileloop) as a proper dependency
- Drop Emacs 26.3 from CI matrix

- actions/checkout v4 -> v6
- actions/stale v9 -> v10

Emacs 25 is ancient at this point and likely no one is using it anymore.

This will fix the following warning:

"The following actions uses node12 which is deprecated and will be
forced to run on node16: actions/checkout@v2. [...]"

Now that Emacs 28.1 is released and the latest stable version available, we should include testing with this version in CI.