LICENSE.txt got 404 replace to LICENSE (#3627)

* fix: link 404

* fix: generate files

Author: yuta-hidaka

BUILD.bazel

@@ -3,7 +3,7 @@ load("@com_github_bazelbuild_buildtools//buildifier:def.bzl", "buildifier")
 load("@io_bazel_rules_go//proto:compiler.bzl", "go_proto_compiler")
 load("@io_bazel_rules_go//proto/wkt:well_known_types.bzl", "PROTO_RUNTIME_DEPS", "WELL_KNOWN_TYPES_APIV2")
 
-exports_files(["LICENSE.txt"])
+exports_files(["LICENSE"])
 
 buildifier(
     name = "buildifier",