feat(v8/browser): Remove XHR transport

[AbhiPrasad]

resolves #5927

Removes makeXHRTransport and makes the fetch transport the default one.

I've also added a warning warning to browser client init that states that the sentry browser sdk requires the fetch API to function.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77.78 KB (-0.17% 🔽)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	69.04 KB (-0.18% 🔽)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	72.97 KB (-0.17% 🔽)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	62.52 KB (-0.33% 🔽)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	33.24 KB (-0.32% 🔽)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	33.15 KB (-0.3% 🔽)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.14 KB (-0.18% 🔽)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31.14 KB (-0.18% 🔽)
@sentry/browser - Webpack (gzipped)	22.4 KB (-0.12% 🔽)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	76.03 KB (-0.3% 🔽)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	67.53 KB (-0.33% 🔽)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	33.49 KB (-0.56% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped)	24.68 KB (-0.43% 🔽)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	212.29 KB (-0.42% 🔽)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	101 KB (-0.78% 🔽)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	73.81 KB (-0.65% 🔽)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	36.79 KB (-0.49% 🔽)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	69.33 KB (-0.19% 🔽)
@sentry/react - Webpack (gzipped)	22.43 KB (-0.17% 🔽)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	86.82 KB (-0.18% 🔽)
@sentry/nextjs Client - Webpack (gzipped)	50.06 KB (-0.22% 🔽)
@sentry-internal/feedback - Webpack (gzipped)	17.16 KB (0%)