.azurepipelines: Use Python 3.8

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=3891

Changes the Python version used in pipelines to 3.8.x (3.8.12) to
workaround an issue discovered after 3.9.10 that causes
stuart_update to fail on a web_dependency (e.g. GCC ARM compilers).

The Python version should be updated to latest once that issue is
root caused and a long term fix is in place. This change is
introduced now to unblock overall CI.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Sean Brogan <sean.brogan@microsoft.com>

.azurepipelines/Ubuntu-PatchCheck.yml

@@ -27,7 +27,7 @@ steps:
 
 - task: UsePythonVersion@0
   inputs:
-    versionSpec: '3.9.x'
+    versionSpec: '3.8.x'
     architecture: 'x64'
 
 - script: |

.azurepipelines/templates/platform-build-run-steps.yml

@@ -42,7 +42,7 @@ steps:
 
 - task: UsePythonVersion@0
   inputs:
-    versionSpec: "3.9.x"
+    versionSpec: "3.8.x"
     architecture: "x64"
 
 - script: pip install -r pip-requirements.txt --upgrade

.azurepipelines/templates/pr-gate-steps.yml

@@ -20,7 +20,7 @@ steps:
 
 - task: UsePythonVersion@0
   inputs:
-    versionSpec: '3.9.x'
+    versionSpec: '3.8.x'
     architecture: 'x64'
 
 - script: pip install -r pip-requirements.txt --upgrade