Remove linkerconfig prop to linker_config

This makes this consistent with its inner props `gen_linker_config` and
`linker_config_srcs`

Test: m nothing --no-skip-soong-tests
Change-Id: I6efee5cc2d182af9c600ec80e3b02d1776ba8eb9
diff --git a/apex/apex_test.go b/apex/apex_test.go
index 9e9efd6..17cea5e 100644
--- a/apex/apex_test.go
+++ b/apex/apex_test.go
@@ -10187,7 +10187,7 @@
 			deps: [
 				"libfoo",
 			],
-			linkerconfig: {
+			linker_config: {
 				gen_linker_config: true,
 				linker_config_srcs: ["linker.config.json"],
 			},