github: Don't build on Ubuntu 18.04 anymore

The Ubuntu 18.04 image is deprecated and builds will start to fail
temporarily during four 24 hour periods from now until the final
deprecation on April 1st.  So better remove these runs now.

.github/workflows/linux.yml

@@ -135,7 +135,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ ubuntu-20.04, ubuntu-18.04 ]
+        os: [ ubuntu-20.04 ]
         test: [ all, nm ]
         compiler: [ gcc, clang ]
         exclude:

scripts/test.sh

@@ -199,11 +199,7 @@ openssl*)
 gcrypt)
 	CONFIG="--disable-defaults --enable-pki --enable-gcrypt --enable-random --enable-pem --enable-pkcs1 --enable-pkcs8 --enable-gcm --enable-hmac --enable-kdf -enable-curve25519 --enable-x509 --enable-constraints"
 	export TESTS_PLUGINS="test-vectors gcrypt! random pem pkcs1 pkcs8 gcm hmac kdf curve25519 x509 constraints"
-	if [ "$ID" = "ubuntu" -a "$VERSION_ID" = "18.04" ]; then
-		DEPS="libgcrypt11-dev"
-	else
-		DEPS="libgcrypt20-dev"
-	fi
+	DEPS="libgcrypt20-dev"
 	;;
 botan)
 	CONFIG="--disable-defaults --enable-pki --enable-botan --enable-pem --enable-hmac --enable-x509 --enable-constraints"
@@ -257,12 +253,7 @@ all|codeql|coverage|sonarcloud)
 		  libmysqlclient-dev libsqlite3-dev clearsilver-dev libfcgi-dev
 		  libldap2-dev libpcsclite-dev libpam0g-dev binutils-dev libnm-dev
 		  libgcrypt20-dev libjson-c-dev python3-pip libtspi-dev libsystemd-dev
-		  libselinux1-dev"
-	if [ "$ID" = "ubuntu" -a "$VERSION_ID" = "18.04" ]; then
-		DEPS="$DEPS iptables-dev python3-setuptools"
-	else
-		DEPS="$DEPS libiptc-dev"
-	fi
+		  libselinux1-dev libiptc-dev"
 	PYDEPS="tox"
 	if test "$1" = "build-deps"; then
 		build_botan
@@ -456,10 +447,7 @@ case "$TEST" in
 		CONFIG="$CONFIG --disable-asan"
 		;;
 	*)
-		if [ "$ID" = "ubuntu" -a "$VERSION_ID" = "18.04" ]; then
-			# the libstdc++ workaround for libbotan doesn't work on Ubuntu 18.04
-			CONFIG="$CONFIG --disable-asan"
-		elif [ "$LEAK_DETECTIVE" != "yes" ]; then
+		if [ "$LEAK_DETECTIVE" != "yes" ]; then
 			CONFIG="$CONFIG --enable-asan"
 		fi
 		;;