Simplify endpoint block execution and deprecate return call in it

[ericproulx]

Deprecate Endpoint Return Value

Summary

This PR simplifies endpoint's block execution. It was relying on an unbound method but it always been re-binded to endpoint instance. Instead, I just call instance_exec from the endpoint instance. First, we deprecate and next version we will remove the rescue LocalJumpError.

Motivation

• I don't know why we want to use return in an endpoint. Seems to be an old thing.
• generate_api_method was generating a couple of strings that were retained in memory. I've seen it in grape-on-rack.

Changes

• Marked the return value of Grape::Endpoint#call as deprecated in the documentation and code comments.
• Added deprecation warnings where appropriate.
• Updated relevant specs and documentation.
• Its a lamba instead of Proc to be able to capture the LocalJumpError and return its exit_value for not breaking anything.

Impact

• No breaking changes for typical API usage.
• Users who were relying on the return value of Grape::Endpoint#call should update their code to avoid this pattern.

Checklist

• Deprecation warning added
• Documentation updated
• Specs updated

[dblock]

Suggested change

	Executing a endpoint's block has been simplified and calling `return` in it has been deprecated.
	Executing a endpoint's block has been simplified and calling `return` in it has been deprecated.
	See [#2577](https://github.com/ruby-grape/grape/pull/2577) for more information.

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Feel free to merge with the UPGRADING update @ericproulx.