Remove usage of List.getFirst() from test (#4375)

getFirst is added in JDK 21, and we are targeting source/binary
compatibility with JDK 17

Author: ScottDugas

fdb-relational-core/src/test/java/com/apple/foundationdb/relational/recordlayer/query/functions/UserDefinedMacroFunctionBuilderTest.java

@@ -85,7 +85,7 @@ void buildFunctionWithSingleStringParameter() {
 
         final var arguments = ImmutableList.of(LiteralValue.ofScalar("hello"));
         assertThat(macroIdentityFunction.encapsulate(arguments))
-                .isEqualTo(arguments.getFirst());
+                .isEqualTo(arguments.get(0));
     }
 
     @Test