Flip experimental_use_coverage_metadata_files (#2830)

Now that the min tested Bazel version is `7.3.1`
(#2812) this flag can
safely be flipped.

Relates to #2082

rust/settings/BUILD.bazel

@@ -82,7 +82,7 @@ bool_flag(
 # reporting tools like `llvm-cov` and `llvm-profdata` as runfiles to each test.
 bool_flag(
     name = "experimental_use_coverage_metadata_files",
-    build_setting_default = False,
+    build_setting_default = True,
 )
 
 # A flag to set rustc --sysroot flag to the sysroot generated by rust_toolchain