Update kotlin samples to be idiomatic

Code example changes:

- Expand outbound adapter examples to show complete `JavaMailSender`
  bean configuration for Java, Java DSL, Kotlin DSL, and Groovy DSL
- Simplify Java examples by inlining return statements
- Add duplicate Groovy DSL example showing both credential styles
- Restructure IMAP idle adapter examples to use inline URI
  configuration instead of separate receiver bean creation
- Remove complete mail flow examples from header enricher section,
  keeping only enrichment examples
- Modify Kotlin DSL examples to use different patterns (`integrationFlow`
  function vs `IntegrationFlow.from()`)
- Update Groovy Properties construction to use map constructor syntax
- Simplify Kotlin `imapMailInboundFlow` to remove explicit return type
- Replace phrasing around terms as "you" and "we"
- Groovy samples were updated but are not idiomatic of the language.  Request assistance

Author: cppwfs