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)

tests/test_conda.py

@@ -21,13 +21,6 @@ def prefix():
             installer_url, checksum
         ) as installer_path:
             conda.install_miniconda(installer_path, tmpdir)
-        conda.ensure_conda_packages(
-            tmpdir,
-            [
-                f"conda=={installer.MAMBAFORGE_CONDA_VERSION}",
-                f"mamba=={installer.MAMBAFORGE_MAMBA_VERSION}",
-            ],
-        )
         yield tmpdir