[Operations] Tighten page load bundle size limits

[mbondyra]

Summary

Hi Operations team! 👋

Recently I've worked on a few PRs that reduce our page load bundle size, and while doing so I noticed that many of our limits are much higher than the actual page load sizes. It made me think: these limits should be realistic, because they’re one of the main ways we protect page load performance.
Right now, based on the current limits, we theoretically allow up to 11,452,104 bytes (~11 MB) to load upfront. But when we check the real page load, it’s only 5.192.979 bytes — about half of that.

We already have a --update-limits flag for the node scripts/build_kibana_platform_plugins.js --update-limits command, but it only bumps limits up when the size grows above the limit, adding a flat 15 KB buffer to the bundle size.
I’d like to propose:

1. Allowing the --update-limits to allow folks to also lower limits when the bundle shrinks drastically
2. Replacing the flat 15 KB buffer with 10% of the plugin’s size, so the buffer scales realistically. Right now, with ~197 plugins, --update-limits allows for an extra (15KB*197=) 3 MB above the total page size — which is way too much in my opinion!

What’s in this PR

✅ Adds logic to let us lower limits automatically, not just bump them up with node scripts/build_kibana_platform_plugins.js --update-limits
✅ Replaces the flat +15 KB bump with a 10% buffer relative to each plugin’s size when using node scripts/build_kibana_platform_plugins.js --update-limits
✅ Updated the limits.yml with the result from the above script

Why it matters

1. Keeps bundle sizes tight
2. Protects us from accidental regressions

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)

[elasticmachine]

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Metrics [docs]

✅ unchanged

History

• 💔 Build #318461 failed 70d0363