refactor(connectivity_plus): remove ReachabilityConnectivityProvider and ReachabilitySwift dependency

[miquelbeltran]

Description

• The min OS versions used are iOS: 12 and macOS: 10.14.
• At the same time, the min OS versions to use the NWPathProvider implementation are also 12 and 10.14.
• The old implementation that users ReachibilitySwift is not being used because the app will always use the NWPathProvider implementation.

Related Issues

• Closes [Request]: Remove Reachability implementation from connectivity_plus ios & macos (darwin)  #2779

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.

[vbuberen]

Nice catch. I remember how this thing caused issues to us some time ago. Glad to see it going.