enlarge test matrix with Racket 8

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        racket-version: ["6.6", "6.7", "6.8", "6.9", "6.10.1", "6.11", "6.12", "7.0", "7.1", "7.2", "7.3", "7.4", "7.5", "7.6", "7.7", "7.8", "7.9", "current"]
+        racket-version: ["6.6", "6.7", "6.8", "6.9", "6.10.1", "6.11", "6.12", "7.0", "7.1", "7.2", "7.3", "7.4", "7.5", "7.6", "7.7", "7.8", "7.9", "8.0", "8.1", "8.2", "8.3", "current"]
         racket-variant: ["BC", "CS"]
         # CS builds are only provided for versions 7.4 and up so avoid
         # running the job for prior versions.