Avoid downgrading user-env conda/mamba

- Only check lower bound on conda/mamba, upgrade unbounded if not matched (let conda apply upper bound according to existing pins, such as Python)
- handle missing mamba
- avoid upgrading Python by aborting the install, instead of keeping old envs
- minimum supported Python for user env is 3.9
- Fix output reporting of conda install step (no need for json capture when we don't parse the output - exit codes will do)

dev-requirements.txt

@@ -1,3 +1,4 @@
+packaging
 pytest
 pytest-cov
 pytest-mock