Use LF line endings in Painless generated files (#26822)

Right now if you run `gradle regen` on Windows you'll get `CRLF` line
endings on all the ANTLR generated files because we run
```
    ant.fixcrlf(srcdir: outputPath) {
      patternset(includes: 'Painless*.java')
    }
```

The docs for fixcrlf say that the default line endings that it
corrects to is based on the OS:
https://ant.apache.org/manual/Tasks/fixcrlf.html

This change locks it to `LF`.

modules/lang-painless/build.gradle

@@ -146,7 +146,7 @@ task regen {
       fileset(dir: outputPath, includes: 'Painless*.java')
     }
     // fix line endings
-    ant.fixcrlf(srcdir: outputPath) {
+    ant.fixcrlf(srcdir: outputPath, eol: 'lf') {
       patternset(includes: 'Painless*.java')
     }
   }