Update Go to 1.26 and bulk update Go dependencies

[simu]

Adjust import paths and define ErrJWTMissing ourselves since the JWT middleware was removed from echo in v4.13.0.

Checklist

• Keep pull requests small so they can be easily reviewed.
• Update the documentation.
• Categorize the PR by setting a good title and adding one of the labels:
  bug, enhancement, documentation, change, breaking, dependency
  as they show up in the changelog

[simu]

Will look into echo/v5 in a follow up PR -- update: oapi-codegen v2.6.0 doesn't support echo v5, so we don't need to look at that right now.

[bastjan]

Added a small cleanup to use the new Go tool directive so we don't need a bogus import for air.