chore(browser): Rename UserAgent integration to HttpContext

[Lms24]

As suggested in this comment of #2906, this PR renames the Browser UserAgent integration to HttpContext. The reason for doing this is that the integration does more than collecting the user agent information and thus the name wasn't reflecting its entire purpose.

Also added a note in the Migration doc about the change.

Sentry-docs PR: getsentry/sentry-docs#4997

ref: https://getsentry.atlassian.net/browse/WEB-270

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	18.7 KB (-7.15% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	58.42 KB (-9.58% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	17.51 KB (-7.16% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	52.6 KB (-9.26% 🔽)
@sentry/browser - Webpack (gzipped + minified)	19.21 KB (-17.35% 🔽)
@sentry/browser - Webpack (minified)	62.08 KB (-24.03% 🔽)
@sentry/react - Webpack (gzipped + minified)	19.23 KB (-17.39% 🔽)
@sentry/nextjs Client - Webpack (gzipped + minified)	42.91 KB (-10.72% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	24.52 KB (-5.98% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	23.1 KB (-5.67% 🔽)