configure: bump version to 0.21.0

Author: apoelstra

configure.ac

@@ -3,8 +3,8 @@ define(_CLIENT_VERSION_MAJOR, 0)
 define(_CLIENT_VERSION_MINOR, 21)
 define(_CLIENT_VERSION_REVISION, 0)
 define(_CLIENT_VERSION_BUILD, 0)
-define(_CLIENT_VERSION_RC, 2)
-define(_CLIENT_VERSION_IS_RELEASE, false)
+define(_CLIENT_VERSION_RC, 0)
+define(_CLIENT_VERSION_IS_RELEASE, true)
 define(_COPYRIGHT_YEAR, 2021)
 define(_COPYRIGHT_HOLDERS,[The %s developers])
 define(_COPYRIGHT_HOLDERS_SUBSTITUTION,[[Elements Project]])