Update mkosi.ubuntu to 'focal'

[zjs: Looking at https://packages.ubuntu.com/bionic/iptables-dev, iptables-dev
was a transitional package that was pulling in libxtables-dev, libip4tc-dev,
and libip6tc-dev (as listed by @GiedriusS). iptables-dev is gone in focal, so
replace it by the expanded list.]

.mkosi/mkosi.ubuntu

@@ -5,7 +5,7 @@
 
 [Distribution]
 Distribution=ubuntu
-Release=bionic
+Release=focal
 Repositories=main,universe
 
 [Output]
@@ -25,7 +25,6 @@ BuildPackages=
         git
         gnu-efi
         gperf
-        iptables-dev
         libacl1-dev
         libaudit-dev
         libblkid-dev
@@ -39,6 +38,8 @@ BuildPackages=
         libgcrypt20-dev
         libgnutls28-dev
         libidn2-0-dev
+        libip4tc-dev
+        libip6tc-dev
         libkmod-dev
         liblz4-dev
         liblz4-tool
@@ -51,6 +52,7 @@ BuildPackages=
         libsmartcols-dev
         libtool
         libxkbcommon-dev
+        libxtables-dev
         libzstd-dev
         m4
         meson
@@ -65,6 +67,6 @@ BuildPackages=
         zstd
 
 Packages=
-        libqrencode3
+        libqrencode4
         locales
         libidn2-0