Update AMI IDs for 6.2.0

[webvictim]

• Use env bash in shebang for MacOS
• Set destination regions using list in Makefile rather than as an independent variable
• Set internal field separator to comma
• Add note about needing coreutils on MacOS
• Add AMI IDs for new regions and reorder to match Makefile list
• Add note about MacOS compatibility
• [auto] Update AMI IDs for 6.2.0

Although #6282 and its backports added support for AMIs in other AWS regions, the script which handles making the AMIs public as part of the Drone promotion step (https://github.com/gravitational/teleport/blob/master/assets/aws/files/make-amis-public.sh) still had a hardcoded region list which was not updated at the same time.

This PR removes that region list and passes it through from the single source of truth in the Makefile instead so this doesn't happen again. I've manually made the affected AMIs public myself now.

This PR also:

• changes the shebang line in AWS-related scripts to use #!/usr/bin/env bash instead of just #/bin/bash so they also work on systems which alter the default path to bash (such as when you brew install bash on MacOS, which goes to /usr/local/bin/bash)
• adds a note about needing to install coreutils, findutils and gnu-sed to run directly on MacOS
  • trying to make scripts compensate for the differences between BSD utils and GNU utils always ends up being a huge pain and leads to one side getting out of sync, so the easiest option is for MacOS users to install GNU tooling (it's better anyway)
  • another option is to run all these scripts inside a VM or Docker, but that seems a bit too heavyweight.

Backports required to:

• branch/v6
• branch/v6.2

Updates #7030

[russjones]

Bot.

[webvictim]

Backports required:

• branch/v6
• branch/v6.2