prettier-plugin-astro: update deprecated pnpm attributes

[joinemm]

Building prettier-plugin-astro with latest nixpkgs unstable results in the following warnings:

trace: evaluation warning: pnpm.configHook: The package attribute is deprecated. Use the top-level pnpmConfigHook attribute instead
trace: evaluation warning: pnpm.fetchDeps: The package attribute is deprecated. Use the top-level fetchPnpmDeps attribute instead

Updated the package definition to use the new attributes, and override it to still force pnpm_9. This fixes the warnings but functionally the build is exactly the same (as this is what nixpkgs would replace them with internally anyway https://github.com/NixOS/nixpkgs/blob/nixos-unstable/pkgs/development/tools/pnpm/generic.nix#L78)

Sanity Checking

• I have updated the changelog as per my changes
• I have tested, and self-reviewed my code
• My changes fit guidelines found in hacking nvf
• Style and consistency
  • I ran Alejandra to format my code (nix fmt)
  • My code conforms to the editorconfig configuration of the project
  • My changes are consistent with the rest of the codebase
• If new changes are particularly complex:
  • My code includes comments in particularly complex areas
  • I have added a section in the manual
  • (For breaking changes) I have included a migration guide
• Package(s) built:
  • .#nix (default package)
  • .#maximal
  • .#docs-html (manual, must build)
  • .#docs-linkcheck (optional, please build if adding links)
• Tested on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin

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[NotAShelf]

Thank you for investigating this :)