remove Fedora 35 support

Fedora 35 is going EOL on Tue 2022-12-13. At the time of writing this commit
message, that's the next day. As we do releases on Wednesdays, the next
release will never find its way to F35 and thus, there's no point in keeping
support for it.

Let's delete everything that relates to Fedora 35. If there's something that
cannot be deleted (e.g. CI containers based on F35), let's upgrade it to F37.

TestCrossArchDepsolve now uses CentOS Stream 8 because RHEL 8.4 cannot read
F37 repository metadata. This is a similar issue to
https://bugzilla.redhat.com/show_bug.cgi?id=2004853 . Basically, newer
repositories can be only read by libmodulemd >= 2.11.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

.github/workflows/tests.yml

@@ -15,7 +15,7 @@ jobs:
     name: "🛃 Unit tests"
     runs-on: ubuntu-20.04
     container:
-      image: registry.fedoraproject.org/fedora:35
+      image: registry.fedoraproject.org/fedora:37
 
     steps:
         # krb5-devel is needed to test internal/upload/koji package
@@ -87,7 +87,7 @@ jobs:
     name: "🐍 Lint (dnf-json)"
     runs-on: ubuntu-latest
     container:
-      image: registry.fedoraproject.org/fedora:35
+      image: registry.fedoraproject.org/fedora:37
     steps:
 
       - name: Install build and test dependencies
@@ -178,7 +178,7 @@ jobs:
   rpmlint:
     name: "📦 RPMlint"
     runs-on: ubuntu-20.04
-    container: registry.fedoraproject.org/fedora:35
+    container: registry.fedoraproject.org/fedora:37
     steps:
       - name: Install dependencies
         run: sudo dnf install -y rpmlint rpm-build make git-core