Add on-call team as codeowner for Dependabot updates (#123)

Add radius-project/on-call as additional codeowner for
.github/workflows/ to allow dependency updates without blocking on
maintainers/approvers.

Signed-off-by: Karishma Chawla <kachawla@microsoft.com>

Author: kachawla

.github/CODEOWNERS

@@ -1,2 +1,5 @@
 # These owners are the maintainers and approvers of this repo
-*       @radius-project/maintainers-design-notes @radius-project/approvers-design-notes
+*       @radius-project/maintainers-design-notes @radius-project/approvers-design-notes
+
+# Files that Dependabot updates - on-call can also approve these
+.github/workflows/      @radius-project/maintainers-design-notes @radius-project/approvers-design-notes @radius-project/on-call