fix(otel): Set root transaction name to be route

[AbhiPrasad]

fixes #6301

If a low cardinality http.route is not defined, we fall back to http.target and set transaction source to be url. In that case, we should special case http.target === / since we know the / root transaction has no parameters. Therefore we categorize it as transaction source route.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.54 KB (-0.01% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	60.42 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.21 KB (+0.03% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	53.76 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	19.94 KB (0%)
@sentry/browser - Webpack (minified)	65.25 KB (0%)
@sentry/react - Webpack (gzipped + minified)	19.96 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	45.96 KB (0%)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.41 KB (-0.01% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.83 KB (+0.01% 🔺)
@sentry/replay index.js	63.38 KB (0%)
@sentry/replay - Webpack (gzipped + minified)	63.39 KB (0%)
@sentry/replay - Webpack (minified)	204.81 KB (0%)