Upgrade ubuntu to 20.04 on mips to have UDP_GRO constant

ci/docker/mips-unknown-linux-gnu/Dockerfile

-# FIXME: Ubuntu 19.10 is missing gcc-mips-linux-gnu.
-FROM ubuntu:19.04
+FROM ubuntu:20.04
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
         gcc libc6-dev qemu-user ca-certificates \

ci/docker/mips64-unknown-linux-gnuabi64/Dockerfile

-FROM ubuntu:19.10
+FROM ubuntu:20.04
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
         gcc libc6-dev qemu-user ca-certificates \

ci/docker/mips64el-unknown-linux-gnuabi64/Dockerfile

-# FIXME: Ubuntu 19.10 is missing gcc-mips-linux-gnu.
-FROM ubuntu:19.04
+FROM ubuntu:20.04
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
         gcc libc6-dev qemu-user ca-certificates \