GH-49609: [C++][R] Replace ursa-labs-r-test bucket with arrow-datasets

The ursa-labs-r-test S3 bucket no longer exists, causing CI failures.
Replace references with the arrow-datasets bucket which is publicly
accessible. Remove the now-redundant RestrictedBucket test in C++ since
it would duplicate the PublicBucket test.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: thisisnic

cpp/src/arrow/filesystem/s3fs_test.cc

@@ -431,12 +431,6 @@ TEST_F(S3RegionResolutionTest, PublicBucket) {
                    ResolveS3BucketRegion("aws-earth-mo-atmospheric-ukv-prd"));
 }
 
-TEST_F(S3RegionResolutionTest, RestrictedBucket) {
-  ASSERT_OK_AND_EQ("us-west-2", ResolveS3BucketRegion("ursa-labs-r-test"));
-  // Same again, cached
-  ASSERT_OK_AND_EQ("us-west-2", ResolveS3BucketRegion("ursa-labs-r-test"));
-}
-
 TEST_F(S3RegionResolutionTest, NonExistentBucket) {
   auto maybe_region = ResolveS3BucketRegion("ursa-labs-nonexistent-bucket");
   ASSERT_RAISES(IOError, maybe_region);