1.28.1 Release

This is a follow up that addresses some issues in the generated code when using access levels on imports.

What's Changed

• Fix Foundation import's access level in generated code by @gjcairo in #1701
• Less generation when no messages, enums, or extensions. by @thomasvl in #1703
• Conditional foundation import by @thomasvl in #1704
• Missed comments on #1704. by @thomasvl in #1705
• Revert "Pin docc to a version that works for older swift versions." by @thomasvl in #1706
• Revise empty file support for import public by @thomasvl in #1707
• Version bump for a release. by @thomasvl in #1708

Full Changelog: 1.28.0...1.28.1