Update links to main branches (#12375)

Author: andrewheard

Firestore/Source/Public/FirebaseFirestore/FIRTimestamp.h

@@ -27,7 +27,7 @@ NS_ASSUME_NONNULL_BEGIN
  * 9999-12-31T23:59:59.999999999Z. By restricting to that range, we ensure that we can convert to
  * and from  RFC 3339 date strings.
  *
- * @see https://github.com/google/protobuf/blob/master/src/google/protobuf/timestamp.proto for the
+ * @see https://github.com/google/protobuf/blob/main/src/google/protobuf/timestamp.proto for the
  * reference timestamp definition.
  */
 NS_SWIFT_NAME(Timestamp)

Firestore/core/include/firebase/firestore/timestamp.h

@@ -38,7 +38,7 @@ namespace firebase {
  * from 0001-01-01T00:00:00Z to 9999-12-31T23:59:59.999999999Z.
  *
  * @see
- * https://github.com/google/protobuf/blob/master/src/google/protobuf/timestamp.proto
+ * https://github.com/google/protobuf/blob/main/src/google/protobuf/timestamp.proto
  */
 class Timestamp {
  public:

Firestore/core/test/unit/FSTGoogleTestTests.mm

@@ -139,7 +139,7 @@ @interface GoogleTests : XCTestCase
  * These members are then joined with a ":" as googletest requires.
  *
  * @see
- * https://github.com/google/googletest/blob/master/googletest/docs/AdvancedGuide.md
+ * https://github.com/google/googletest/blob/main/docs/advanced.md
  */
 NSString* CreateTestFiltersFromTestsToRun(NSSet<NSString*>* testsToRun) {
   NSMutableString* result = [[NSMutableString alloc] init];

scripts/update_xcode_target.rb

@@ -15,7 +15,7 @@
 # limitations under the License.
 
 # Script to add a file to an Xcode target.
-# Adapted from https://github.com/firebase/quickstart-ios/blob/master/scripts/info_script.rb
+# Adapted from https://github.com/firebase/quickstart-ios/blob/main/scripts/info_script.rb
 
 require 'xcodeproj'
 project_path = ARGV[0]