JS: Deprecate getUnknownMember() and replace its uses with getArrayElement()

Although they mean slightly different things, every single call site
of getUnknownMember() just used it as a way to get array elements.

Since there is no known use-case for the original meaning of
getUnknownMember() I am deprecating it for now.

javascript/ql/src/experimental/Security/CWE-347/JWT.qll

@@ -7,7 +7,7 @@ DataFlow::Node unverifiedDecode() {
     verify
         .getParameter(2)
         .getMember("algorithms")
-        .getUnknownMember()
+        .getArrayElement()
         .asSink()
         .mayHaveStringValue("none") and
     result = verify.getParameter(0).asSink()
@@ -32,7 +32,7 @@ DataFlow::Node verifiedDecode() {
       not verify
           .getParameter(2)
           .getMember("algorithms")
-          .getUnknownMember()
+          .getArrayElement()
           .asSink()
           .mayHaveStringValue("none") or
       not exists(verify.getParameter(2).getMember("algorithms"))

javascript/ql/src/experimental/semmle/javascript/Execa.qll

@@ -72,7 +72,7 @@ module Execa {
     override predicate isShellInterpreted(DataFlow::Node arg) {
       // if shell: true then first and second args are sinks
       // options can be third argument
-      arg = [this.getArgument(0), this.getParameter(1).getUnknownMember().asSink()] and
+      arg = [this.getArgument(0), this.getParameter(1).getArrayElement().asSink()] and
       isExecaShellEnable(this.getParameter(2))
       or
       // options can be second argument