Temporarily revert use of pattern match instanceof until docs-reference is fixed

Temporarily revert use of pattern match instanceof construct until docs-reference is fixed, see JDK-8258657.

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Generating REFERENCE_API javadoc for 21 modules
/Users/chhegar/git/open/src/java.net.http/share/classes/jdk/internal/net/http/HttpClientImpl.java:161: error: pattern matching in instanceof is a preview feature and is disabled by default.
            if (delegate instanceof ExecutorService service) {
                                                    ^
  (use --enable-preview to enable pattern matching in instanceof)
1 error
make[3]: *** [/Users/chhegar/git/open/build/macosx-x64/support/docs/_javadoc_REFERENCE_API_exec.marker] Error 1
Docs.gmk:472: recipe for target '/Users/chhegar/git/open/build/macosx-x64/support/docs/_javadoc_REFERENCE_API_exec.marker' failed
make/Main.gmk:485: recipe for target 'docs-reference-api-javadoc' failed
make[2]: *** [docs-reference-api-javadoc] Error 2
make[2]: *** Waiting for unfinished jobs....