Workaround failing fmt compilation by locking to v6.2.1

Summary: Updating pods attempts to bump fmt to 7.1.3, which causes CircleCI to fail. [Sample failure](https://app.circleci.com/pipelines/github/facebook/react-native/9422/workflows/f4c9076a-9649-490c-b565-555fecc60957/jobs/205827). Let's temporarily lock to an older version until the [fmt fix](355be4b13f) gets tagged for release.

Reviewed By: JoshuaGross

Differential Revision: D29147585

fbshipit-source-id: b1aca9618586a9d6e4c6d0c2c37b258745e008ee

packages/rn-tester/Podfile.lock

@@ -77,18 +77,18 @@ PODS:
   - RCT-Folly (2021.04.26.00):
     - boost-for-react-native
     - DoubleConversion
-    - fmt
+    - fmt (~> 6.2.1)
     - glog
     - RCT-Folly/Default (= 2021.04.26.00)
   - RCT-Folly/Default (2021.04.26.00):
     - boost-for-react-native
     - DoubleConversion
-    - fmt
+    - fmt (~> 6.2.1)
     - glog
   - RCT-Folly/Fabric (2021.04.26.00):
     - boost-for-react-native
     - DoubleConversion
-    - fmt
+    - fmt (~> 6.2.1)
     - glog
   - RCTRequired (1000.0.0)
   - RCTTypeSafety (1000.0.0):
@@ -877,7 +877,7 @@ SPEC CHECKSUMS:
   glog: 5337263514dd6f09803962437687240c5dc39aa4
   libevent: 4049cae6c81cdb3654a443be001fb9bdceff7913
   OpenSSL-Universal: 1aa4f6a6ee7256b83db99ec1ccdaa80d10f9af9b
-  RCT-Folly: 8250e510cd9fde2f292ad258682264d4a1d38bff
+  RCT-Folly: 0dd9e1eb86348ecab5ba76f910b56f4b5fef3c46
   RCTRequired: af2d6080a4b9ba0885b28ca78879a92066c71cab
   RCTTypeSafety: f5405e0143bb2addae97ce33e4c87d9284a48fe2
   React: f64c9f6db5428717922a3292ba6a448615a2e143

third-party-podspecs/RCT-Folly.podspec

@@ -17,7 +17,7 @@ Pod::Spec.new do |spec|
   spec.dependency 'boost-for-react-native'
   spec.dependency 'DoubleConversion'
   spec.dependency 'glog'
-  spec.dependency 'fmt'
+  spec.dependency 'fmt' , '~> 6.2.1'
   spec.compiler_flags = '-DFOLLY_NO_CONFIG -DFOLLY_MOBILE=1 -DFOLLY_USE_LIBCPP=1 -DFOLLY_HAVE_PTHREAD=1 -Wno-comma -Wno-shorten-64-to-32 -Wno-documentation'
   spec.source_files = 'folly/String.cpp',
                       'folly/Conv.cpp',