Update docker configuration (ROS and o3de)

[jhanca-robotecai]

It was double-checked that the demo works correctly in docker container with iron installed.

The changes are following:

• update default ROS distro in Dockerfile to iron
• remove the comment about non-existing iron docker in readme
• update default o3de and o3de-extras branches from stabilization into official releases of the same versions

[jhanca-robotecai]

Fixes #250

[michalpelka]

Asking @spham-amzn for acceptance. Thanks!