Update ios_facts.py (#40928)

Account for upper/lower case match occurrences of "[Nn]umber" and "[Ss]erial"

Model Number                       : WS-C3850-12X48U
System Serial Number          :  <removed>

lib/ansible/modules/network/ios/ios_facts.py

@@ -204,11 +204,11 @@ class Default(FactsBase):
             return match.group(1)
 
     def parse_stacks(self, data):
-        match = re.findall(r'^Model number\s+: (\S+)', data, re.M)
+        match = re.findall(r'^Model [Nn]umber\s+: (\S+)', data, re.M)
         if match:
             self.facts['stacked_models'] = match
 
-        match = re.findall(r'^System serial number\s+: (\S+)', data, re.M)
+        match = re.findall(r'^System [Ss]erial [Nn]umber\s+: (\S+)', data, re.M)
         if match:
             self.facts['stacked_serialnums'] = match