[core] update version number

NOTE: this is purposely version 0.21, despite the release goal is 1.0
Since we issue weekly builds, people might otherwise misunderstand the version

So first when the release goal is reached, FreeCAD becomes version 1.0

.github/ISSUE_TEMPLATE/BUG-REPORT.yml

@@ -29,8 +29,8 @@ body:
       label: Version
       description: What version of FreeCAD are you running?
       options:
-        - 0.19 (Release)
-        - 0.20 (Development)
+        - 0.20 (Release)
+        - 0.21 (Development)
     validations:
       required: true
   - type: textarea