Update snios minimum os version to iOS 15

[charliescheer]

Fix

We are going to update the minimum supported version of iOS for Simplenote to iOS 15.

This PR sets those minimums and then fixes all the warnings that cropped up after making those changes.

Test

This changes a bunch of things, so a general app smoke test would be a good idea. Somethings that definitely need to be confirmed:

• All three Simplenote widgets look right on iOS 17
• viewing markdown preview still works
• sharing from other apps to simplenote still works
• sharing from the notes app Bear using Text Bundle as the export type works

Review

(Required) Add instructions for reviewers. For example:

Only one developer are required to review these changes, but anyone can perform the review.

[wpmobilebot]

You can test the changes in simplenote-ios from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr1548-b64127c-018d23c0-4c45-4da5-a680-75a6b7a9a278 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[charliescheer]

Converting back to draft cause I realized the @available version checking wasn't removed yet

[charliescheer]

Okay, this is finally ready now

[peril-automattic]

	Warnings
⚠️	PR is not assigned to a milestone.

Generated by 🚫 dangerJS

[jleandroperez]

✅ All test scenarios verified
✅ Changes look beautiful

Nice work sir!!!