delete "migrating to spring 3.x" sections

Issue: SPR-11171
Mention suffix pattern match in contentnegotiationmanager doc

Issue: SPR-11172
Mention RestTemplate updates in documentation

Issue: SPR-11173
update interceptors documentation

Issue: SPR-11175
Polish
Polish
Polish
Polish
polish
Polish
Add indent=0 and extra tabs to source blocks

Ensure tabs are converted to spaces