[cron] Bump distribution date (2021-12-26)

Marlin/Version.h

  * here we define this default string as the date where the latest release
  * version was tagged.
  */
-//#define STRING_DISTRIBUTION_DATE "2021-12-25"
+//#define STRING_DISTRIBUTION_DATE "2021-12-26"
 
 /**
  * Defines a generic printer name to be output to the LCD after booting Marlin.

Marlin/src/inc/Version.h

  * version was tagged.
  */
 #ifndef STRING_DISTRIBUTION_DATE
-  #define STRING_DISTRIBUTION_DATE "2021-12-25"
+  #define STRING_DISTRIBUTION_DATE "2021-12-26"
 #endif
 
 /**