ref(feedback): Change form box-shadow to use CSS var

[billyvg]

This makes it consistent with the buttons box shadow, so they both have the same box shadow by default.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	65.62 KB (0%)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	55.73 KB (0%)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.02 KB (0%)
@sentry/browser - Webpack (gzipped)	21.33 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	62.05 KB (0%)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	29.14 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped)	21.27 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	195.75 KB (0%)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	88.42 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	63.38 KB (0%)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.86 KB (0%)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	65.93 KB (0%)
@sentry/react - Webpack (gzipped)	21.37 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	82.67 KB (0%)
@sentry/nextjs Client - Webpack (gzipped)	48.16 KB (0%)
@sentry-internal/feedback - Webpack (gzipped)	16.09 KB (-0.12% 🔽)