chore: Deprecate partially supported system config (canonical#5515)

ds-identify does not support the fs_label key. This key is only partially
supported.

Deprecate it. Users of custom labels may switch to cidata or CIDATA.

Note: Tools such as cloud-localds hard-code the label.

cloudinit/sources/DataSourceNoCloud.py

@@ -130,6 +130,12 @@ def _pp2d_callback(mp, data):
 
         label = self.ds_cfg.get("fs_label", "cidata")
         if label is not None:
+            if label.lower() != "cidata":
+                util.deprecate(
+                    deprecated="Custom fs_label keys",
+                    deprecated_version="24.3",
+                    extra_message="This key isn't supported by ds-identify.",
+                )
             for dev in self._get_devices(label):
                 try:
                     LOG.debug("Attempting to use data from %s", dev)

doc/examples/cloud-config-datasources.txt

@@ -34,9 +34,6 @@ datasource:
     # seedfrom: http://my.example.com/i-abcde/
     seedfrom: None
 
-    # fs_label: the label on filesystems to be searched for NoCloud source
-    fs_label: cidata
-
     # these are optional, but allow you to basically provide a datasource
     # right here
     user-data: |