ci: update to rust version of flint

[zeitlinger]

This updates the existing link linting to the current Rust-based flint release.

What changes in this PR:

• install flint from github:grafana/flint = "0.20.3"
• replace the old remote lint:links shell task with flint run
• keep the existing Lychee configuration under .github/config
• enable check_all_local = true so local links are checked across the repo
• preserve --full on push and scheduled runs so link checking still scans the full repo outside pull requests

Why this stays small:

• this repo does not currently carry Markdown lint wiring to migrate here
• the goal is only to replace the old link-check task with the released Rust flint path

Verification:

• mise run lint
• mise run lint --full

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.31%. Comparing base (df8063b) to head (9281a36).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8315   +/-   ##
=========================================
  Coverage     90.31%   90.31%           
  Complexity     7725     7725           
=========================================
  Files           850      850           
  Lines         23259    23259           
  Branches       2364     2364           
=========================================
  Hits          21007    21007           
  Misses         1528     1528           
  Partials        724      724           

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[jack-berg]

What do all these new linters do? Regardless, let's keep this PR focussed on updating to the new version and separate out any change in functionality to separate PRs.

[zeitlinger]

Yes that was the idea - a little bit too much slipped through and should be fixed now