update CI to use ubuntu-latest and test Nim 2.0

Author: Vindaar

.github/workflows/ci.yml

@@ -18,11 +18,11 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        branch: [version-1-6, devel]
+        branch: [version-1-6, version-2-0, devel]
         target: [linux] #, macos, windows]
         include:
           - target: linux
-            builder: ubuntu-18.04
+            builder: ubuntu-latest
         #  - target: macos
         #    builder: macos-10.15
         #  - target: windows