Update tooling (Ruby, Pods, Deployment Targets)

[AliSoftware]

Checklist

• [x] I've checked that all new and existing tests pass
• [x] I've updated the documentation if necessary
• [x] I've added an entry in the CHANGELOG to credit myself

Description

• Add a Gemfile (Same as in https://github.com/AliSoftware/OHHTTPStubs/pull/367)
• Add .ruby-version file (Ruby 3.2.2)
• Switch Podfile to use CocoaPods' CDN (instead of master repo)
• Update deployment target versions in .podspec and Podfiles for Xcode 15.2+ minimums
• Xcode: Update to recommended settings
• Remove deprecated NSURLConnection calls in tests
• Update Examples (Podfile, NSURLConnection -> NSURLSession, …)
• Fix some CHANGELOG entries

cc @Liquidsoul I just noticed you already implemented some of this in https://github.com/AliSoftware/OHHTTPStubs/pull/367 and https://github.com/AliSoftware/OHHTTPStubs/pull/366; I only saw those branches after I started doing the work, but figured I'd finish my work and submit my version of the PR containing everything.