Error on use of self intead of Self in return type

[esdrubal]

Description

With this PR we no longer support using self instead of Self when referring to the Self Type.

Fixes #6396

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[codspeed-hq]

CodSpeed Performance Report

Merging #6763 will not alter performance

_{Comparing esdrubal/6396 (56920a6) with master (bcd15c3)}

Summary

✅ 22 untouched benchmarks