[Fuchsia] Update fuchsia_target_api_level to 15

Bug: https://github.com/dart-lang/sdk/issues/38752
Change-Id: I7c951a66776747abc5bb756b2360649e5daa2265
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/333940
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Zijie He <zijiehe@google.com>
diff --git a/.gn b/.gn
index b38467d..7639dab 100644
--- a/.gn
+++ b/.gn
@@ -17,10 +17,5 @@
 script_executable = "python3"
 
 default_args = {
-  # fuchsia::io::InotifyWatchMask has been deprecated in 12 and removed in 13
-  # right before the current version (12.20230418.0.1) we are using.
-  # https://fxrev.dev/836377.
-  # So explicitly marking fuchsia_target_api_level to 11 to workaround it
-  # temporarily.
-  fuchsia_target_api_level = 11
+  fuchsia_target_api_level = 15
 }