.azurepipelines: Fix cspell version to v5.20.0

The v5.21.0 update introduced a number of new issues that might
impact stable tag integration so this fixes the version to the
previous release.

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: Liming Gao <gaoliming@byosoft.com.cn>

This value can be revisited in the future to update the

.azurepipelines/templates/spell-check-prereq-steps.yml

@@ -17,6 +17,6 @@ steps:
     #checkLatest: false # Optional
   condition: and(gt(variables.pkg_count, 0), succeeded())
 
-- script: npm install -g cspell
+- script: npm install -g cspell@5.20.0
   displayName: 'Install cspell npm'
   condition: and(gt(variables.pkg_count, 0), succeeded())