Feat: Update Minimum Release Version and Remove Deprecated Code

[BrandonStalnaker]

Background

• Our next major release will update the minimum release version to 15.6. In addition to removing code that is no longer necessary we also wanted to use this opportunity to remove all uses deprecated of deprecated code as well. Saving AppDelegate proxy and related methods https://rokt.atlassian.net/browse/SDKE-553

What Has Changed

• Updated minimum release version to 15.6 and removed longstanding deprecated code.

Checklist

• I have performed a self-review of my own code.
• [] I have made corresponding changes to the documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have tested this locally.

Additional Notes

• {Any additional information or context relevant to this PR}

Reference Issue (For employees only. Ignore if you are an outside contributor)

• Closes https://rokt.atlassian.net/browse/SDKE-554

[nickolas-dimitrakas]

LGTM

[denischilik]

A lot of great cleanup and modernization work is done here, but the PR is quite large and very difficult to review safely. Some parts also appear to modify behavior, not just update deployment targets or remove deprecated APIs.

To make the review clearer and safer, could we split this into three focused PRs:

1. deployment target and podspec updates,
2. removal of deprecated public APIs,
3. internal logic updates (persistence, networking, utilities)?

[jamesnrokt]

Nit: Should these prints be going to logger instead?

[BrandonStalnaker]

I broke this into three commits and corresponding PRs based off Denis' request. Thanks for looking @denischilik @nickolas-dimitrakas @jamesnrokt @rmi22186

I'm working on James' nit now but I've gotta inject the logger dependency into MPUserDefaults.swift and that gets initialized in a bunch of odd places so it may take a little time (thankfully its in the 3rd of the 3 prs)

feat: Update Minimum Deployment Targets
feat: Remove Deprecated Public APIs
refactor: Update internal logic

The merge-base changed after approval.