INT-3492: Deprecate `@Paylod` and `@Header(s)`

JIRA: https://jira.spring.io/browse/INT-3492

Since `@Paylod` and `@Header(s)` annotations are in the Spring Framework,
there is no more need to support them in Spring Integration.

Deprecate them and leave for backward compatibility.
Will be removed in future releases.

Remove deprecated Gatewa's `#method` expression evaluation context variable