CI: Switch to M1 medium workers

These are twice as costly per usage minute.
macos.x86.medium.gen2 will be gone in June 2024,
so eventually we will need to switch anyway.
Maybe the raw performance makes up for the increase in costs.

.circleci/config.yml

@@ -478,7 +478,7 @@ jobs:
   Check Swift formatting:
     macos:
       xcode: "15.0"
-    resource_class: "macos.x86.medium.gen2"
+    resource_class: "macos.m1.medium.gen1"
     steps:
       - checkout
       - run:
@@ -500,7 +500,7 @@ jobs:
   iOS build and test:
     macos:
       xcode: "15.0"
-    resource_class: "macos.x86.medium.gen2"
+    resource_class: "macos.m1.medium.gen1"
     steps:
       - checkout
       - run:
@@ -607,7 +607,7 @@ jobs:
   iOS integration test:
     macos:
       xcode: "15.0"
-    resource_class: "macos.x86.medium.gen2"
+    resource_class: "macos.m1.medium.gen1"
     steps:
       - checkout
       - skip-if-doc-only
@@ -665,7 +665,7 @@ jobs:
   iOS Framework release:
     macos:
       xcode: "15.0"
-    resource_class: "macos.x86.medium.gen2"
+    resource_class: "macos.m1.medium.gen1"
     steps:
       - checkout
       - attach_workspace:
@@ -885,7 +885,7 @@ jobs:
   pypi-macos-release:
     macos:
       xcode: "15.0"
-    resource_class: "macos.x86.medium.gen2"
+    resource_class: "macos.m1.medium.gen1"
     steps:
       - install-rustup
       - setup-rust-toolchain