Remove the Rspack production assets feature flags

What does this MR do?

Removes the rspack_production_assets and rspack_production_assets_opt_in feature flags. Both have been fully enabled on gitlab.com production since 2026-10-16.

Webpack is no longer supported for production assets:

• Every production install, including self-managed, serves Rspack assets (manifest.rspack.json). ENABLE_RSPACK=false is ignored in production, and rolling back to Webpack means reverting this MR.
• yarn build (run by gitlab:assets:compile) always builds with Rspack.
• In development and test, the manifest still follows webpack.bundler in gitlab.yml (and ENABLE_RSPACK), because GDK supports Webpack until 16 November 2026. The Webpack config stays for that.

CI now builds assets with Rspack only:

• compile-production-assets and its variants (as-if-foss, istanbul, vue3) build with Rspack. The separate compile-production-assets-rspack jobs and cache:assets-hash-rspack are gone.
• build-assets-image packs a single Rspack build instead of merging Webpack and Rspack output.
• compile-production-assets partial-cache is removed. It filled a Webpack build cache, and Rspack production builds don't use a persistent cache.
• bundle-size-review is removed (job, rules, scripts/bundle_size_review, danger/Dangerfile-bundle_size, docs mention). It measured a Webpack build, so its numbers no longer describe what ships. MRs won't get the bundle-size Danger comment until an Rspack version exists: #635715.
• CI no longer sets ENABLE_RSPACK, GLCI_ASSETS_BUNDLER or WEBPACK_VENDOR_DLL. The partial assets cache used by compile-test-assets is keyed on config/rspack.config.mjs instead of config/webpack.config.js.
• The assets hash no longer includes a bundler suffix, so the first pipeline after merge recompiles assets once.

References

• Feature issue: #604382 (closed)
• Rollout issues: #606363 (closed) and #631217 (closed)
• Remove Webpack: #604386
• Rsdoctor bundle size report: #635715

Reproduction steps

1. In this MR's pipeline, compile-production-assets logs an Rspack build, and there are no compile-production-assets-rspack or bundle-size-review jobs.
2. On a production-mode instance built from this branch (RAILS_ENV=production bin/rake gitlab:assets:compile), public/assets/webpack/manifest.rspack.json exists and pages load assets from it, even with ENABLE_RSPACK=false.

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