[meta] Update pm2

[philippemilink]

The current version with Node 24 raises warnings at start:

% npm run server
> zmarkdown@12.1.0 server
> pm2 start -f server/index.js -i 3 --max-memory-restart 150M

(node:269514) [DEP0060] DeprecationWarning: The `util._extend` API is deprecated. Please use Object.assign() instead.
(Use `node --trace-deprecation ...` to show where the warning was created)
[PM2] Spawning PM2 daemon with pm2_home=/home/philippe/.pm2
[PM2] PM2 Successfully daemonized
(node:269514) [DEP0044] DeprecationWarning: The `util.isArray` API is deprecated. Please use `Array.isArray()` instead.
[PM2] Starting /media/linux/dev/zds/zmarkdown/packages/zmarkdown/server/index.js in cluster_mode (3 instances)
(node:269514) [DEP0176] DeprecationWarning: fs.R_OK is deprecated, use fs.constants.R_OK instead
[PM2] Done.

Updating pm2 dependency fixes the problem.