AB#8098 AT_ -> TRAKKA_ env vars. Deprecation warnings for AT_

[samcarswell]

• Now accepting TRAKKA_ env vars; these take precedence over AT_.
• Any AT_ env vars in the environment that are not overridden by the corresponding TRAKKA_ will produce a deprecation warning
• Use of austrakka-admin for [TRAKKA|AT]_CMD_SET will produce a deprecation warning; this occurs before logger initialisation so it will look a little funny. Details are in the comments in cmd_filter.py.
• Fixed occurrences in the test code where the combined stdout/stderr from a command was parsed into json rather than just the stdout; printing a bunch of warnings to stderr surfaced it

PR Checklist

• Ran test suite locally
• PR to update development docs if needed
• PR to update user docs including changelog if needed

Double check the repo merge order here
if you have companion PRs in other repositories.