Make CI Linkcheck green again

[marxin]

After the recent effort, one can run full Linkcheck locally and all links are valid now.

However, the GitHub Action CI is still unhappy for 2 reasons:

• Server returned 429 Too Many Requests for - these are a known limitation and the effect of the linkcheck cache that is not persisted and reused (I've got a WIP changes: Save linkcheck cache always #2030)

• Server returned 403/500 Internal Server Error for - these errors are very weird and I don't have a clue what can cause that:

error: Server returned 500 Internal Server Error for https://github.com/rust-lang/rust/pulls?q=is%3Apr+label%3AS-inactive+is%3Aclosed
    ┌─ getting-started.md:145:67
    │
145 │ these PRs and pick up the work. You can find the list of such PRs [here][abandoned-prs].
    │                                                                   ^^^^^^^^^^^^^^^^^^^^^ Server returned 500 Internal Server Error for https://github.com/rust-lang/rust/pulls?q=is%3Apr+label%3AS-inactive+is%3Aclosed

error: Server returned 403 Forbidden for https://github.com/bors
   ┌─ about-this-guide.md:77:7
   │
77 │ - For [@bors], [this cheat sheet][cheatsheet] is helpful
   │       ^^^^^^^ Server returned 403 Forbidden for https://github.com/bors

error: Server returned 403 Forbidden for https://github.com/bors
  ┌─ tests/ci.md:9:32
  │
9 │ - When the PR is approved, the [bors] bot enqueues the PR into a [merge queue].
  │                                ^^^^^^ Server returned 403 Forbidden for https://github.com/bors

error: Server returned 403 Forbidden for https://github.com/bors
    ┌─ tests/ci.md:1[30](https://github.com/rust-lang/rustc-dev-guide/actions/runs/10133701540/job/28019147907#step:7:31):46
    │
130 │ all PRs have to be approved through our bot, [bors] (the software behind it is
    │                                              ^^^^^^ Server returned 403 Forbidden for https://github.com/bors

[marxin]

I investigated that Server returned 403 Forbidden for https://github.com/bors errors are caused by legacy openssl package being used by mdbook-linkcheck. The package update is needed here: Michael-F-Bryan/mdbook-linkcheck#82.

[marxin]

CC: @camelid

[marxin]

I would say the CI is pretty reliable and there are no broken links at the time of writing this post.