Update displayed country codes of flags

Update the country codes which are displayed for the flags in the UI. This only changes the names which are displayed in the UI but not how flags are communicated between server and client.

- Use ISO 3166-2 subdivision codes:
   - England: XEN → GB-ENG
   - Northern Ireland: XNI → GB-NIR
   - Scotland: XSC → GB-SCT
   - Wales: XWA → GB-WLS
   - Catalonia: XCA → ES-CT
   - Galicia: XGL → ES-GA
- Use ISO 3166/MA exceptional reservation code:
   - European Union: XEU → EU
- Move South Sudan (SS) to official codes, as it was officially assigned in the year 2011.

Closes #7071.

CMakeLists.txt

@@ -1222,8 +1222,11 @@ set(EXPECTED_DATA
   countryflags/EG.png
   countryflags/EH.png
   countryflags/ER.png
+  countryflags/ES-CT.png
+  countryflags/ES-GA.png
   countryflags/ES.png
   countryflags/ET.png
+  countryflags/EU.png
   countryflags/FI.png
   countryflags/FJ.png
   countryflags/FK.png
@@ -1231,6 +1234,10 @@ set(EXPECTED_DATA
   countryflags/FO.png
   countryflags/FR.png
   countryflags/GA.png
+  countryflags/GB-ENG.png
+  countryflags/GB-NIR.png
+  countryflags/GB-SCT.png
+  countryflags/GB-WLS.png
   countryflags/GB.png
   countryflags/GD.png
   countryflags/GE.png
@@ -1396,13 +1403,6 @@ set(EXPECTED_DATA
   countryflags/VU.png
   countryflags/WF.png
   countryflags/WS.png
-  countryflags/XCA.png
-  countryflags/XEN.png
-  countryflags/XEU.png
-  countryflags/XGL.png
-  countryflags/XNI.png
-  countryflags/XSC.png
-  countryflags/XWA.png
   countryflags/YE.png
   countryflags/ZA.png
   countryflags/ZM.png

data/countryflags/index.txt

@@ -6,30 +6,30 @@
 default
 == -1
 
-XEN
+##### ISO 3166-2 subdivisions #####
+
+GB-ENG
 == 901
 
-XNI
+GB-NIR
 == 902
 
-XSC
+GB-SCT
 == 903
 
-XWA
+GB-WLS
 == 904
 
-XEU
-== 905
-
-XCA
+ES-CT
 == 906
 
-XGL
+ES-GA
 == 907
 
-#south sudan, non official code#
-SS
-== 737
+##### ISO 3166/MA exceptional reservations #####
+
+EU
+== 905
 
 ##### ISO 3166-1 based #####
 
@@ -648,6 +648,9 @@ SB
 SO
 == 706
 
+SS
+== 737
+
 ZA
 == 710