Drop iOS 14 Support: Fix an issue with PostMetaButton not visible from Interface Builder

[kean]

Related Issue #20860

I was doing regression testing for my iOS 14-related changes and found an issue in the Reader.

Turns out after changing PostMetaButton from Objective-C to Swift in #20897, it stopped being visible from the Interface Builder. In this PR, I revert this change for now until there is a better solution.

Note: I enabled auto-merge to make sure it gets merge quickly

To test:

• Open Reader
• Verify that post button are displayed correctly

Regression Notes

1. Potential unintended areas of impact: n/a
2. What I did to test those areas of impact (or what existing automated tests I relied on): n/a
3. What automated tests I added (or what prevented me from doing so): n/a

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

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📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

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