Revert "UI: Change default format for release builds to fMP4/fMOV"

This reverts commit 9488bb20b6.

This is currently a frequent support issue as Windows Media Player,
Movies & TV, and File Explorer all cannot seem to handle fMP4 files
correctly. Windows Media Player and Movies & TV will play the files
fine, but no duration is listed and the applications cannot seek the
file. File Explorer will simply not list the file's length/duration (the
field is shown as having no set value). This was somehow not caught
during a year's worth of beta and RC builds. The world just wasn't ready
for this. =(

We have reported these issues to Microsoft, and we will revisit this in
the future.

(cherry picked from commit 22205d582c)

UI/window-basic-main.cpp

@@ -1462,8 +1462,9 @@ static const double scaled_vals[] = {1.0,         1.25, (1.0 / 0.75), 1.5,
 				     2.5,         2.75, 3.0,          0.0};
 
 extern void CheckExistingCookieId();
-
-#ifdef __APPLE__
+#if OBS_RELEASE_CANDIDATE == 0 && OBS_BETA == 0
+#define DEFAULT_CONTAINER "mkv"
+#elif defined(__APPLE__)
 #define DEFAULT_CONTAINER "fragmented_mov"
 #else
 #define DEFAULT_CONTAINER "fragmented_mp4"