ci: update to rust version of flint

[zeitlinger]

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

What changes in this PR:

• install flint from github:grafana/flint = "0.20.3"
• run the existing markdown and link checks through flint instead of the old remote shell task plus separate markdownlint wiring
• keep the existing lychee configuration under .github/config
• enable check_all_local = true so local links are checked across the repo

Why markdown-lint-check is now captured:

• markdownlint-cli2 was already part of this repo
• once flint is added, it discovers and runs markdownlint-cli2 from mise.toml
• that means the old dedicated markdownlint workflow and shell task would be duplicate CI logic
• this PR removes that duplicate path and lets the existing markdown lint run through the same flint-based lint entrypoint as link checking

This is the smaller first step. The broader flint migration remains in the follow-up PR.

Verification:

• mise run lint

[trask]

is this because link checks are now part of lint? can we split it out so that link checks aren't a required status check? or are you thinking that with the link check only applying to diffs it shouldn't cause PRs to get blocked?

[zeitlinger]

is this because link checks are now part of lint?

yes

can we split it out so that link checks aren't a required status check?

in flint that's currently not possible - but could be added

or are you thinking that with the link check only applying to diffs it shouldn't cause PRs to get blocked?

yes - that was the idea

[trask]

ok, let's try it

cc @laurit