Drop 1.7 CI. Limit concurrency to leave CI space for org (#764)

Author: IanButterworth

.github/workflows/test.yml

@@ -17,11 +17,11 @@ jobs:
     timeout-minutes: 90
     runs-on: ${{ matrix.os }}
     strategy:
+      max-parallel: 20 # leave space for other runs in the JuliaLang org, given these tests are long
       fail-fast: false
       matrix:
         julia-version:
           - '1.6'
-          - '1.7'
           - '1.8'
           - '1.9-nightly'
           - 'nightly'