Bump Roundcube to version 1.6.13 (#5222)

Security fixes:

    Fix CSS injection vulnerability reported by CERT Polska.
    Fix remote image blocking bypass via SVG content reported by nullcathedral.

install/upgrade/upgrade.conf

@@ -50,7 +50,7 @@ pga_v='7.14.6'
 
 # Set version of RoundCube (Webmail) to update during upgrade if not already installed
 # Note: only applies to "non-apt installs >= 1.4.0 or manually phased out"
-rc_v='1.6.12'
+rc_v='1.6.13'
 
 # Set version of SnappyMail (Webmail) to update during upgrade if not already installed
 sm_v='2.38.2'