Put Browser session-logic into integration and deprecate `autoSessionTracking`

[lforst]

Description

As pre-work for #14478 we want to put the logic for sessions in the browser into an integration and deprecate autoSessionTracking.

The upside of putting the logic into an integration is better treeshakability, better interfacability when using a custom browser client, and configurability.

Tasks:

• Expose integration for browser (feat(browser): Add browserSessionIntegration #14551)
• Add migration path to http integration for server (feat(node): Add trackIncomingRequestsAsSessions option to http integration #14567)
• Update docs to refer to integrations instead of autoSessionTracking (Add docs for browserSessionIntegration and trackIncomingRequestsAsSessions as alternative to autoSessionTracking in JS SDKs sentry-docs#12056)
• Deprecate autoSessionTracking (feat: Deprecate autoSessionTracking #14640)