fix: Remove Depricated Mailgun distribution from the SDK docs

[Arindam200]

Description:

In our current SDK, there are multiple instances of Mailgun distribution that have been deprecated. This outdated information may confuse users and hinder their integration efforts. We need to promptly remove all mentions of the deprecated method and update the documentation to reflect current practices.

Tasks:

1. Identify all instances of deprecated Mailgun distribution within the SDK codebase.
2. Replace deprecated Mailgun distribution with recommended alternatives or remove them entirely if no longer needed.
3. Update documentation and relevant references to reflect the removal of deprecated Mailgun distribution.
4. Perform thorough testing to ensure that the removal does not introduce any regressions or issues.