Upgrade legacy build from Buster to Bookworm

[bmorelli25]

Summary

Legacy doc builds have been broken since Friday when Buster reached EOL. Attempt to upgrade to Bookworm to fix the build.

https://github.com/elastic/docs/blob/master/Dockerfile#L1

https://elastic.slack.com/archives/C05PP2LEC1X/p1752498514766499

[bmorelli25]

Pin to Buster if possible

[cotti]

Fixed by elastic/docs#3241