Use `delegate` for Jwt methods

What does this MR do and why?

Describe in detail what your merge request does and why.

Use delegate for Jwt methods

Refactor the many delegation methods so that the code is more compact.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

In the Rails console:

build = Ci::Build.last
Gitlab::Ci::Jwt.new(build, ttl: 1.hour).payload
Gitlab::Ci::JwtV2.new(build, aud: 'my-aud', ttl: 1.hour).payload

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.