Bug 1654607: Upgrades glean in bootstrap r=firefox-build-system-reviewers,rstewart

A newer version of Glean is needed to take advantage of the recent "optional version" change.

Pins glean to 31.5.0 to avoid breaking changes, such as to the YAML schema.

Differential Revision: https://phabricator.services.mozilla.com/D84566

python/mozboot/mozboot/bootstrap.py

@@ -887,14 +887,9 @@ def _install_glean():
     If the current python instance is a virtualenv, then glean is installed
     directly.
     If not, then glean is installed to the Python user install directory.
+    Upgrades glean if it's out-of-date.
     """
-    try:
-        import glean  # noqa: F401
-        return  # Glean is already installed, we can skip the installation
-    except ImportError:
-        pass
-
-    pip_call = [sys.executable, '-m', 'pip', 'install', 'glean_sdk']
+    pip_call = [sys.executable, '-m', 'pip', 'install', 'glean_sdk~=31.5.0']
     if not os.environ.get('VIRTUAL_ENV'):
         # If the user is already using a virtual environment before they invoked
         # `mach bootstrap`, then we shouldn't add the "--user" flag. This is because