Revert to Ubuntu 20.10 (#6678)

Till #6591 is fixed.

frameworks/C/nginx/nginx.dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:21.04
+FROM ubuntu:20.10
 
 ARG DEBIAN_FRONTEND=noninteractive
 

frameworks/PHP/amp/amp.dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:21.04
+FROM ubuntu:20.10
 
 ARG DEBIAN_FRONTEND=noninteractive
 

frameworks/PHP/reactphp/reactphp.dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:21.04
+FROM ubuntu:20.10
 
 ARG DEBIAN_FRONTEND=noninteractive