app: fix #14312 on first run copies settings from 2.10 instead of 3.0

I noticed when running 3.1.2 for the first time it copied settings
from 2.10 instead of 3.0 as I would expect.

Investigation shows that the major version number is hardcoded to 2,
so let's change that to major.
In addition to that, the outer loop continues running even when we
found a migration path if there is a lower major version number
available. Let's change this so we stop when migrate is TRUE.

Also fix some issues that could hit us in the future:
- Change the value of `major` in the for loop from a hardcoded value
to using `GIMP_MAJOR_VERSION`.
- The flatpak specific testing is for version 2.10 but only tested
the minor version. Change that to make sure major is 2.

Author: Wormnest

app/core/gimp-user-install.c

@@ -297,7 +297,7 @@ user_install_detect_old (GimpUserInstall *install,
       gint major;
       gint minor;
 
-      for (major = 3; major >= 2; major--)
+      for (major = GIMP_MAJOR_VERSION; major >= 2; major--)
         {
           gint max_minor;
 
@@ -324,14 +324,14 @@ user_install_detect_old (GimpUserInstall *install,
 
               if (migrate)
                 {
-                  install->old_major = 2;
+                  install->old_major = major;
                   install->old_minor = minor;
 
                   break;
                 }
 
 #ifdef G_OS_UNIX
-              if (minor == 10)
+              if (major == 2 && minor == 10)
                 {
                   /* This is special-casing for GIMP 2.10 as flatpak where
                    * we had this weird inconsistency: depending on whether a
@@ -381,6 +381,8 @@ user_install_detect_old (GimpUserInstall *install,
                 }
 #endif
             }
+          if (migrate)
+            break;
         }
     }