CI: use build_meta.json instead of build_meta.txt.

[worldomonation]

Related to #73557.

Proposed Changes

This PR migrates all builds to produce a build_meta.json instead of the build_meta.txt, completing the missing task in #73557.

Testing Instructions

Calypso App builds should produce a build_meta.json.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-ajp-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-58378

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-58378&env=jetpack

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[noahtallen]

should we base this on webpackConfig.output instead of having a separate variable? (same for happy-blocks)