Fix Reader detail headers with featured images on iOS 15

[frosty]

This PR fixes an issue where Reader Detail navigation bars for posts with featured images were white, which meant that buttons on the bar weren't visible.

Before	After

To test

• Build and run
• In Reader, head to Discover and find a post with a featured image at the top. Ensure that the navigation bar is clear and you can see the buttons.
• Test on iOS 14 as well as 15 to check everything still looks as it should.
• Double check some other areas of the Reader to see that this hasn't broken anything else there.

Regression Notes

1. Potential unintended areas of impact

Other navigation bars in the Reader.

2. What I did to test those areas of impact (or what existing automated tests I relied on)

I manually checked through various sections of the Reader to see that the navigation bar still looked as I expected.

3. What automated tests I added (or what prevented me from doing so)

None

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.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 4 days Please, make sure to get it merged by then or assign it to a later expiring milestone

Generated by 🚫 dangerJS

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[leandroalonso]

[AliSoftware]

Does this means that if a post has a featured image which happens to be mostly white (at least at the top part of the image), the NavBar buttons won't be visible either, because of the translucent appearance?

[leandroalonso]

@AliSoftware yes. I'd consider that an edge case though. However, if the user scrolls or tries the swipe-to-back gesture it will work.

Anyway, this was the behavior before this issue.

[AliSoftware]

Yeah I just tested as well on my end, and even scrolling a couple of pixels will make the buttons appear again anyway. So

[AliSoftware]

This fix just got shipped into the new 18.8.0.3 on TestFlight 🎉