Update email_spec to 2.2.0

What does this MR do?

Updates email_spec from 1.6.0 to 2.2.0, and accordingly removes the workaround named as have_html_escaped_body_text (!10015 (merged) (#20121 (closed))).

Are there points in the code the reviewer needs to double check?

Diff: https://github.com/email-spec/email-spec/compare/v1.6.0...v2.2.0 and Changelog https://github.com/email-spec/email-spec/blob/v2.2.0/Changelog.md are available.

Why was this MR needed?

To officially support Rails 4 and Ruby 2.3/2.4.

The upstream support similar with have_html_escaped_body_text can be adapted by updating this dependency. If it had been updated earlier, !10015 (merged) would not have been introduced 😄

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them

What are the relevant issue numbers?

Closes #46845 (closed)