lavc: bump minor and add Changelog entry for the Vulkan H265 encoder

Changelog

@@ -22,6 +22,7 @@ version <next>:
 - YUV colorspace negotiation for codecs and filters, obsoleting the
   YUVJ pixel format
 - Vulkan H.264 encoder
+- Vulkan H.265 encoder
 
 
 version 7.0:

libavcodec/version.h

@@ -29,7 +29,7 @@
 
 #include "version_major.h"
 
-#define LIBAVCODEC_VERSION_MINOR  14
+#define LIBAVCODEC_VERSION_MINOR  15
 #define LIBAVCODEC_VERSION_MICRO 100
 
 #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \