Bump Geany ABI to mitigate Scintilla 5.1.5 changes

This is to mitigate this change in Scintilla 5.1.5:

When calling SCI_GETTEXT, SCI_GETSELTEXT, and SCI_GETCURLINE with a NULL
buffer argument to discover the length that should be allocated, do not
include the terminating NUL in the returned value. The value returned
is 1 less than previous versions of Scintilla. Applications should
allocate a buffer 1 more than this to accommodate the NUL. The wParam
(length) argument to SCI_GETTEXT and SCI_GETCURLINE also omits the NUL.
This is more consistent with other APIs.

While it doesn't prevent simple recompilation of existing plugins
without fixing the possible problems in them, at least it doesn't allow
users using binary releases to load old, unfixed plugins.

src/plugindata.h

@@ -68,7 +68,7 @@ G_BEGIN_DECLS
  * Changing this forces all plugins to be recompiled before Geany can load them. */
 /* This should usually stay the same if fields are only appended, assuming only pointers to
  * structs and not structs themselves are declared by plugins. */
-#define GEANY_ABI_VERSION (72 << GEANY_ABI_SHIFT)
+#define GEANY_ABI_VERSION (73 << GEANY_ABI_SHIFT)
 
 
 /** Defines a function to check the plugin is safe to load.