feat: cease using internet_connection_checker

[gkc]

Closes #292 . This PR will complete one of the tasks identified in #292 ; the other two tasks are completed by the changes in this at_client PR which have now been released in version 3.0.64 of the at_client package

- What I did
feat: cease using internet_connection_checker

- How I did it

• use latest version of at_client, v3.0.64, which eliminates all usage within the at_client package itself of the internet_connection_checker package
• removed usage of internet_connection_checker package by removing usage of at_client's now-deprecated ConnectivityListener
• replaced with a simple periodic heartbeat check

- How to verify it

• automated tests pass
• run sshnpd; take down your connection to internet; should see a message connection lost within at most 25 seconds; bring the internet connection back up; should see a message connection available within 15-20 seconds

- Description for the changelog
feat: cease using internet_connection_checker

[cconstab]

We might in the future want to add an option to set the polling value from 15-120 seconds. Beyond 120 second the TCP stack will kick in and drop things.

Or even to switch this off to save bandwidth...