[IS-7.0 Migration] Add migration docs for token type handling in custom token issuers

[KD23243]

Is your suggestion related to an experience ? Please describe.
The migration documents needs to account for changes introduced in token type handling and token renewal processes for custom token issuers.

Describe the improvement

• Remove references to the type = "jwt" in the Deployment.toml configuration previously used for specifying token types, as it is no longer required.

  [[oauth.extensions.token_types]]
  name = "CustomTokenIssuer"
  type = "jwt"
  issuer = "org.wso2.carbon.identity.extensions.CustomJWTTokenIssuer"
  persist_access_token_alias = true
  

• Emphasize that custom JWT token issuers that do not extend the JWTTokenIssuer class must override the new getAccessTokenType() method to specify the token type explicitly.

• Provide a guideline for implementations of the OauthTokenIssuer interface to override the getAccessTokenType() method for compatibility.

Additional context

• Related Issues:
  • Property "renew_token_without_revoking_existing" is not being honored causing stuck threads #20994
  • JWT token performance issue when renew_token_without_revoking_existing is enabled #21122
  • "renew_token_without_revoking_existing" not working for custom jwt token issuers #21355
• Related PRs
  • Property "renew_token_without_revoking_existing" not being honored causing stuck threads wso2-extensions/identity-inbound-auth-oauth#2630