CI: Bump the runner to ubuntu-20.04

The ubuntu-20.04 runner is now the oldest Ubuntu version supported by
hosted runners. Elixir 1.6.6 testing has been replaced with 1.10 to
match Mint's CI.

Author: the-mikedavis

.github/workflows/main.yml

@@ -7,8 +7,7 @@ on:
 jobs:
   test:
     name: Test (Elixir ${{ matrix.elixir }}, OTP ${{ matrix.erlang }})
-    # We need Ubuntu 18.04 for older OTP versions.
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix:
@@ -24,7 +23,7 @@ jobs:
           - erlang: "23.3.1"
             elixir: "1.11.4"
           - erlang: "21.3"
-            elixir: "1.6.6"
+            elixir: "1.10"
     env:
       GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       MIX_ENV: test