[2.x] ci(core): allow pnpm to be used as package manager

[DavideIadeluca]

2.x equivalent to #4910

Fixes #0000
flarum/action-build has support for using pnpm as the package manager since many years, however the reusable frontend workflow never added full support for it.

Changes proposed in this pull request:

• Adds a additional CI step which installs pnpm if it's the provided package manager
• Bumps node because the latest pnpm version requires node version 22 or newer
• Bumps actions/setup-node to further get rid of some warnings

Reviewers should focus on:

• pnpm needs to be installed by us in comparison to yarn and npm because those package managers are installed by default on GitHub hosted runners on Ubuntu Latest: https://github.com/actions/runner-images/blob/main/images/ubuntu/Ubuntu2404-Readme.md
• pnpm will refuse to install the dependencies when the package.json is missing the packageManager field. A documentation PR for this should be done

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)