github: Move CI to ubuntu 20.04 base image.

18.04 image is deprecated and will disappear soon.  Also some
slowdowns and brownouts are planned to push users away from
this deprecated version:

  actions/runner-images#6002

Moving to 20.04.  Can't move to 22.04 at the moment because of
deprecation warnings from openssl 3.0.

Acked-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

.github/workflows/build-and-test.yml

@@ -6,7 +6,7 @@ jobs:
   build-linux:
     env:
       dependencies: |
-        automake libtool gcc bc libjemalloc1 libjemalloc-dev    \
+        automake libtool gcc bc libjemalloc2 libjemalloc-dev    \
         libssl-dev llvm-dev libelf-dev libnuma-dev libpcap-dev  \
         ninja-build selinux-policy-dev
       AFXDP:       ${{ matrix.afxdp }}
@@ -22,7 +22,7 @@ jobs:
       TESTSUITE:   ${{ matrix.testsuite }}
 
     name: linux ${{ join(matrix.*, ' ') }}
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     timeout-minutes: 30
 
     strategy: