opal-ci: Drop ubuntu-20.04, add ubuntu-24.04

Standard support for Ubuntu 20.04 ended on May 31, 2025. Remove it and
add Ubuntu 24.04.

Signed-off-by: Reza Arbab <arbab@linux.ibm.com>
diff --git a/.github/workflows/docker-builds-checks.yml b/.github/workflows/docker-builds-checks.yml
index 3a26df1..22fdbc8 100644
--- a/.github/workflows/docker-builds-checks.yml
+++ b/.github/workflows/docker-builds-checks.yml
@@ -12,7 +12,7 @@
       fail-fast: false
 
       matrix:
-        os: [ ubuntu-20.04, ubuntu-22.04, ubuntu-rolling, fedora41, fedora42, fedora-rawhide, docs ]
+        os: [ ubuntu-22.04, ubuntu-24.04, ubuntu-rolling, fedora41, fedora42, fedora-rawhide, docs ]
 
     steps:
       - uses: actions/checkout@v4
diff --git a/opal-ci/Dockerfile-ubuntu-20.04 b/opal-ci/Dockerfile-ubuntu-24.04
similarity index 78%
rename from opal-ci/Dockerfile-ubuntu-20.04
rename to opal-ci/Dockerfile-ubuntu-24.04
index 8333b12..18d4d83 100644
--- a/opal-ci/Dockerfile-ubuntu-20.04
+++ b/opal-ci/Dockerfile-ubuntu-24.04
@@ -1,9 +1,9 @@
-FROM ubuntu:20.04
+FROM ubuntu:24.04
 ENV DEBIAN_FRONTEND    noninteractive
 RUN apt-get update -qq
 RUN if [ `arch` != "ppc64le" ]; then apt-get install -y gcc-powerpc64le-linux-gnu; fi
 RUN apt-get install -y gcc-arm-linux-gnueabi || true
-RUN apt-get install -y gcc ccache expect libssl-dev wget curl xterm device-tree-compiler build-essential gcc python g++ pkg-config libz-dev libglib2.0-dev libpixman-1-dev libfdt-dev git libstdc++6 valgrind libtcl8.6 libmbedtls-dev
+RUN apt-get install -y gcc ccache expect libssl-dev wget curl xterm device-tree-compiler build-essential gcc python3 g++ pkg-config libz-dev libglib2.0-dev libpixman-1-dev libfdt-dev git libstdc++6 valgrind libtcl8.6 libmbedtls-dev
 RUN if [ `arch` = "x86_64" ]; then curl -O https://public.dhe.ibm.com/software/server/powerfuncsim/p9/packages/v1.1-0/systemsim-p9-1.1-0-trusty_amd64.deb; dpkg -i systemsim-p9-1.1-0-trusty_amd64.deb; fi
 RUN if [ `arch` = "x86_64" ]; then curl -O https://public.dhe.ibm.com/software/server/powerfuncsim/p10/packages/v1.2-1/ubuntu2004/systemsim-p10_1.2-1_amd64.deb; dpkg -i systemsim-p10_1.2-1_amd64.deb; fi
 COPY . /build/
diff --git a/opal-ci/build-ubuntu-20.04.sh b/opal-ci/build-ubuntu-24.04.sh
similarity index 94%
rename from opal-ci/build-ubuntu-20.04.sh
rename to opal-ci/build-ubuntu-24.04.sh
index 48eb825..ba070b4 100755
--- a/opal-ci/build-ubuntu-20.04.sh
+++ b/opal-ci/build-ubuntu-24.04.sh
@@ -4,7 +4,7 @@
 set -e
 set -vx
 
-MAKE_J=$(grep -c processor /proc/cpuinfo)
+MAKE_J=$(nproc)
 
 export CROSS="ccache powerpc64le-linux-gnu-"