Update dependency `pulldown-cmark` to v0.12.0 for `mdman`

[weihanglo]

Problem

Renovate bot found a newer version available for dependency pulldown-cmark in https://github.com/rust-lang/cargo/pull/14623. pulldown-cmark is used by mdman for templatizing and generating man pages.

However, the new version has some API changes that need to be fixed:

Build failure. Click to See details

``` error[E0532]: expected unit struct, unit variant or constant, found tuple variant `TagEnd::BlockQuote` --> crates/mdman/src/format/man.rs:266:25 | 2[66](https://github.com/rust-lang/cargo/actions/runs/11116205209/job/30886065074?pr=14623#step:16:67) | TagEnd::BlockQuote => { | ^^^^^^^^^^^^^^^^^^ help: use the tuple variant pattern syntax instead: `TagEnd::BlockQuote(_)` | ::: /Users/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pulldown-cmark-0.12.1/src/lib.rs:256:5 | 256 | BlockQuote(Option), | ---------- `TagEnd::BlockQuote` defined here error[E0532]: expected unit struct, unit variant or constant, found tuple variant `TagEnd::BlockQuote` --> crates/mdman/src/format/text.rs:234:21 | 234 | TagEnd::BlockQuote => { | ^^^^^^^^^^^^^^^^^^ help: use the tuple variant pattern syntax instead: `TagEnd::BlockQuote(_)` | ::: /Users/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pulldown-cmark-0.12.1/src/lib.rs:256:5 | 256 | BlockQuote(Option), | ---------- `TagEnd::BlockQuote` defined here For more information about this error, try `rustc --explain E0532`. ```

Steps

1. Clone rust-lang/cargo
2. Run cargo build-man
3. Fix the build failure.

Possible Solution(s)

Look into what have been changed in pulldown-cmark and update accordingly.

Version

15fbd2f607d4defc87053b8b76bf5038f2483cf4

[dacianpascu06]

I will claim this issue.

[dacianpascu06]

@rustbot claim