Update XCode version in CircleCI (#1803)

Current version is being deprecated:

https://circleci.com/changelog/deprecation-of-xcode-14-3-1-26-3-0-26-2-0-and-26-1-1/

.circleci/config.yml

diff --git a/.circleci/config.yml b/.circleci/config.yml
index a4c2463..9ca4462 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -14,9 +14,11 @@
       # brew itself which takes more than 4 minutes.
       HOMEBREW_NO_AUTO_UPDATE: "1"
     macos:
-      # Corresponds to macOS 13.2.1
+      # Corresponds to macOS 14.3.1
       # See https://circleci.com/docs/guides/execution-managed/using-macos/#supported-xcode-versions
-      xcode: "14.3.1"
+      # Note that we still build our binaries targeting an older version (currently MacOS 11) but
+      # CI providers do not support OS versions that old.
+      xcode: "15.4.0"
     resource_class: m4pro.medium
   linux_arm64:
     machine: