fix(ci): drop hardcoded pnpm version, defer to packageManager

[JohnMcLear]

Summary

The release run that fired after PR #1 merged failed at Install pnpm:

Error: Multiple versions of pnpm specified:

• version 10 in the GitHub Action config with the key "version"
• version pnpm@11.0.6 in the package.json with the key "packageManager"

The publish job checks out ether/etherpad-lite (pinned to pnpm@11 via packageManager) into cwd, then pnpm/action-setup@v6 errors on the version mismatch.

This is the same fix that landed in ep_plugin_helpers#12. Without version: action-setup takes the version from packageManager (now pinned to pnpm@11.0.6 in this plugin's package.json too).

After this lands the release job will reach the publish step and ep_syntax_highlighting@0.1.x will appear on npm.

Test plan

• Same fix exercised cleanly in ep_plugin_helpers#12 → produced 0.5.1 on npm.
• Will be exercised here on merge.

🤖 Generated with Claude Code

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