[2.x] perf: update Babel browserslist targets to reduce bundle size

[rafaucau]

This PR updates the browser targets in babel.config.cjs:

'>0.2%, not dead, not android > 0, not operamini all'

to:

'>0.5%, last 2 versions, not dead, not operamini all'

The new query raises the global usage threshold while explicitly retaining the latest two versions of each browser. This drops support for low-usage legacy browser versions and reduces unnecessary Babel transforms, while keeping broad compatibility with current browsers.

The explicit Android exclusion is also no longer necessary, since unsupported legacy versions are excluded by the remaining criteria.

Bundle size impact

Rebuilding flarum/core produced the following results:

Asset	Before	After	Saving
dist/forum.js	476 KB	440 KB	36 KB
dist/admin.js	464 KB	428 KB	36 KB

Because flarum-webpack-config is shared across the ecosystem, extensions using this configuration can benefit from the updated targets when rebuilt.

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Ref: https://discuss.flarum.org/d/39653-are-we-supporting-too-old-browser