ci: partially replace Ubuntu 18.04 on GHA with Ubuntu 20.04

GitHub started the deprecation process for Ubuntu 18.04 Action
runners on 2022-08-08, and Ubuntu 18.04 will be completely
unsupported by 2022-12-01 (if things proceed as planned).

See <https://github.blog/changelog/2022-08-09-github-actions-the-ubuntu-18-04-actions-runner-image-is-being-deprecated-and-will-be-removed-by-12-1-22/>
for the announcement and more information on that.

So this commit replace those Ubuntu 18.04 runners with the
next newest Ubuntu 20.04 runner that have shown to still work in
the draft PR #716.

The other five GHA jobs with Ubuntu 18.04 (gcc-6, gcc-7,
clang++-3.9, clang++-4.0, clang++-5.0) still need to be dealt
with separately later.

.github/workflows/ci.yml

@@ -28,14 +28,16 @@ jobs:
           - toolset: gcc
             compiler: g++-8
             cxxstd: "14"
-            os: ubuntu-18.04
+            os: ubuntu-20.04
             install: g++-8
           - toolset: gcc-9
             cxxstd: "14,17"
-            os: ubuntu-18.04
+            os: ubuntu-20.04
+            install: g++-9
           - toolset: gcc-10
             cxxstd: "14,17"
-            os: ubuntu-18.04
+            os: ubuntu-20.04
+            install: g++-10
           - toolset: clang
             compiler: clang++-3.9
             cxxstd: "14"
@@ -54,12 +56,12 @@ jobs:
           - toolset: clang
             compiler: clang++-6.0
             cxxstd: "14,17"
-            os: ubuntu-18.04
+            os: ubuntu-20.04
             install: clang-6.0
           - toolset: clang
             compiler: clang++-7
             cxxstd: "14,17"
-            os: ubuntu-18.04
+            os: ubuntu-20.04
             install: clang-7
           - toolset: clang
             compiler: clang++-8