feat(feedback): [v7] Customize feedback placeholder text color

[c298lee]

Backports #11417 to v7

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	80.55 KB (+0.1% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	71.52 KB (+0.08% 🔺)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	75.54 KB (+0.08% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	65.16 KB (+0.09% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	35.46 KB (+0.15% 🔺)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	35.34 KB (+0.14% 🔺)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.54 KB (+0.08% 🔺)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31.55 KB (+0.08% 🔺)
@sentry/browser - Webpack (gzipped)	22.74 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	78.67 KB (+0.11% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	70.02 KB (+0.08% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	35.8 KB (+0.1% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	25.18 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	220.42 KB (+0.06% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	108.23 KB (+0.11% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	75.49 KB (+0.01% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	39.03 KB (+0.1% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	72.01 KB (+0.07% 🔺)
@sentry/react - Webpack (gzipped)	22.78 KB (+0.04% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	89.85 KB (+0.05% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	53.99 KB (+0.08% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	17.32 KB (+0.1% 🔺)