Remove compatibility with Nim versions < 1.6

The `check eventually` template is incompatible
with Nim 1.2. It uses `await` in a template,
which is not possible until after 1.2.

nim-lang/Nim#12085 (comment)

Author: markspanbroek

.github/workflows/test.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        nim: [stable, 1.2.6]
+        nim: [stable, 1.6.12]
         os: [ubuntu-latest, macOS-latest, windows-latest]
     steps:
     - uses: actions/checkout@v2