docs(apache-notice.md): add a notice acknowledging the use of GTS config settings

This might actually be unnecessary; since I've copied over configuration settings from the GTS package, I figured I'd add this notice. It's in a file, since there's no capacity for adding comments in a JSON or .rc file. It feels doubtful that configuration settings fall under the auspices of the Apache License, but I'll leave that to the maintainers to decide.

Author: dancrumb

packages/grpc-js/apache-notice.md

@@ -0,0 +1,4 @@
+The following files contain configuration settings that were derived from [the this commit](https://github.com/google/gts/commit/3b9ab6dd59691f77f5c5c632a44c6762ba4ef7c6) in the Google GTS repository:
+ - .eslintrc
+ - prettier.config.js
+ - tsconfig.json