Removed usage of obsolete ubuntu-20.04 GHA image.

Author: Lastique

.github/workflows/ci.yml

@@ -1,4 +1,4 @@
-# Copyright 2023-2024 Andrey Semashev
+# Copyright 2023-2025 Andrey Semashev
 #
 # Distributed under the Boost Software License, Version 1.0.
 # https://www.boost.org/LICENSE_1_0.txt
@@ -72,12 +72,14 @@ jobs:
               - g++-8
           - toolset: gcc-9
             cxxstd: "11,14,17,2a"
-            os: ubuntu-20.04
+            os: ubuntu-latest
+            container: ubuntu:20.04
             install:
               - g++-9
           - toolset: gcc-10
             cxxstd: "11,14,17,20"
-            os: ubuntu-20.04
+            os: ubuntu-latest
+            container: ubuntu:20.04
             install:
               - g++-10
           - toolset: gcc-11
@@ -186,13 +188,15 @@ jobs:
           - toolset: clang
             compiler: clang++-9
             cxxstd: "11,14,17,2a"
-            os: ubuntu-20.04
+            os: ubuntu-latest
+            container: ubuntu:20.04
             install:
               - clang-9
           - toolset: clang
             compiler: clang++-10
             cxxstd: "11,14,17,20"
-            os: ubuntu-20.04
+            os: ubuntu-latest
+            container: ubuntu:20.04
             install:
               - clang-10
           - toolset: clang