Use Ubuntu 18.04 for all CI tasks that previously used Ubuntu 20.04

This is to prevent linking with a version of glibc that is too recent.

With meilisearch v0.29.0 we inadvertently bumped the minimum supported
glibc version to 2.29, which means it couldn't be run from Debian 10
(for example) anymore. By using Ubuntu 18.04, which uses glibc 2.27, we
restore support for older Linux distros.

.github/workflows/publish-binaries.yml

@@ -38,9 +38,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-20.04, macos-latest, windows-latest]
+        os: [ubuntu-18.04, macos-latest, windows-latest]
         include:
-          - os: ubuntu-20.04
+          - os: ubuntu-18.04
             artifact_name: meilisearch
             asset_name: meilisearch-linux-amd64
           - os: macos-latest
@@ -77,7 +77,7 @@ jobs:
       matrix:
         include:
           - build: aarch64
-            os: ubuntu-20.04
+            os: ubuntu-18.04
             target: aarch64-unknown-linux-gnu
             linker: gcc-aarch64-linux-gnu
             use-cross: true