Require JDK 21, not 17

The NFA compiler switches over a sealed hierarchy of records. Pattern
matching for switch was still a preview feature in 17 and was only
finalised in 21, so the JDK 17 leg of CI could not compile it. Caught by
the build rather than by a reader, which is what the matrix is for.

Author: luccahp1

.github/workflows/ci.yml

@@ -13,7 +13,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        java: ['17', '21', '24']
+        # 21 is the floor: the NFA compiler switches over a sealed hierarchy, and
+        # pattern matching for switch was still a preview feature in 17.
+        java: ['21', '24']
     name: JDK ${{ matrix.java }}
     steps:
       - uses: actions/checkout@v4

README.md

@@ -118,7 +118,8 @@ step is never revisited.
 
 ## Build and test
 
-Requires JDK 17 or newer. There is no Maven or Gradle to install.
+Requires JDK 21 or newer. The NFA compiler switches over a sealed hierarchy of records, and
+pattern matching for `switch` was only finalised in 21. There is no Maven or Gradle to install.
 
 ```bash
 javac -d out $(find src test -name '*.java')