CI: Move most arm64 jobs to experimental pipeline

We've seen significant delays in the arm pipeline lately, with some changes
waiting nine or more hours for jobs to run. We should do what we can to
alleviate demand for arm64 nodes; I'm not sure any other projects have four
arm jobs on every change.

Change-Id: I7929ec3b856d90201c4cc4bf201e634446d3ad88

.zuul.yaml

@@ -606,16 +606,18 @@
       enable_fips: true
 
 - project-template:
-   name: swift-jobs-arm64
-   description: |
-     Runs tests for an OpenStack Python project under the CPython
-     version 3 releases designated for testing on top of ARM64 architecture.
-   check-arm64:
-     jobs:
-       - swift-tox-py38-arm64
-       - swift-tox-py312-arm64
-       - swift-tox-func-encryption-py38-arm64
-       - swift-tox-func-py38-arm64
+    name: swift-jobs-arm64
+    description: |
+      Runs tests for an OpenStack Python project under the CPython
+      version 3 releases designated for testing on top of ARM64 architecture.
+    check-arm64:
+      jobs:
+        - swift-tox-py312-arm64
+    experimental:
+      jobs:
+        - swift-tox-py38-arm64
+        - swift-tox-func-encryption-py38-arm64
+        - swift-tox-func-py38-arm64
 
 - project:
     templates: