[BUG] KeyError when using s3fs backend on 3006.8

Description
After upgrading from 3006.7 to 3006.8 on the salt-master, the following logs are seen in the master log:

2024-05-02 15:38:03,398 [salt.loaded.int.fileserver.s3fs:406 ][DEBUG   ][26069] Refreshing buckets cache file
2024-05-02 15:38:03,399 [salt.loaded.int.utils.s3:163 ][DEBUG   ][26069] S3 Request: http://my-bucket.s3.us-gov-east-1.amazonaws.com/?marker=
2024-05-02 15:38:03,399 [salt.loaded.int.utils.s3:164 ][DEBUG   ][26069] S3 Headers::
2024-05-02 15:38:03,399 [salt.loaded.int.utils.s3:165 ][DEBUG   ][26069]     Authorization: AWS4-HMAC-SHA256 Credential=XXX/20240502/us-gov-east-1/s3/aws4_request, SignedHeaders=host;x-amz-content-sha256;x-amz-date;x-amz-security-token, Signature=XXX
2024-05-02 15:38:03,400 [urllib3.connectionpool:246 ][DEBUG   ][26069] Starting new HTTP connection (1): my-bucket.s3.us-gov-east-1.amazonaws.com:80
2024-05-02 15:38:03,451 [urllib3.connectionpool:474 ][DEBUG   ][26069] http://my-bucket.s3.us-gov-east-1.amazonaws.com:80 "GET /?marker= HTTP/1.1" 200 None
2024-05-02 15:38:03,452 [salt.loaded.int.utils.s3:228 ][DEBUG   ][26069] S3 Response Status Code: 200
2024-05-02 15:38:03,453 [salt.loaded.int.fileserver.s3fs:636 ][DEBUG   ][26069] Writing buckets cache file
2024-05-02 15:38:03,453 [salt.master      :469 ][ERROR   ][26069] Uncaught exception while updating s3fs fileserver cache
Traceback (most recent call last):
  File "/opt/saltstack/salt/lib/python3.10/site-packages/salt/master.py", line 467, in _do_update
    update_func(*args)
  File "/opt/saltstack/salt/lib/python3.10/site-packages/salt/loader/lazy.py", line 159, in __call__
    ret = self.loader.run(run_func, *args, **kwargs)
  File "/opt/saltstack/salt/lib/python3.10/site-packages/salt/loader/lazy.py", line 1245, in run
    return self._last_context.run(self._run_as, _func_or_method, *args, **kwargs)
  File "/opt/saltstack/salt/lib/python3.10/site-packages/salt/loader/lazy.py", line 1260, in _run_as
    return _func_or_method(*args, **kwargs)
  File "/opt/saltstack/salt/lib/python3.10/site-packages/salt/fileserver/s3fs.py", line 125, in update
    metadata = _init()
  File "/opt/saltstack/salt/lib/python3.10/site-packages/salt/fileserver/s3fs.py", line 371, in _init
    _prune_deleted_files(metadata)
  File "/opt/saltstack/salt/lib/python3.10/site-packages/salt/fileserver/s3fs.py", line 594, in _prune_deleted_files
    cached_files.add(meta["Key"])
KeyError: 'Key'

Setup

fileserver_backend:
  - roots
  - gitfs
  - s3fs
s3.buckets:
  - my-bucket
s3.service_url: s3.us-gov-east-1.amazonaws.com
ext_pillar_first: True
ext_pillar:
  - s3:
      bucket: my-bucket
      service_url: s3.us-gov-east-1.amazonaws.com
      prefix: pillar

Versions Report

Salt Version:
          Salt: 3006.8

Python Version:
        Python: 3.10.14 (main, Apr  3 2024, 21:30:09) [GCC 11.2.0]

Dependency Versions:
          cffi: 1.14.6
      cherrypy: unknown
      dateutil: 2.8.1
     docker-py: Not Installed
         gitdb: Not Installed
     gitpython: Not Installed
        Jinja2: 3.1.3
       libgit2: 1.6.4
  looseversion: 1.0.2
      M2Crypto: Not Installed
          Mako: Not Installed
       msgpack: 1.0.2
  msgpack-pure: Not Installed
  mysql-python: Not Installed
     packaging: 22.0
     pycparser: 2.21
      pycrypto: Not Installed
  pycryptodome: 3.19.1
        pygit2: 1.12.2
  python-gnupg: 0.4.8
        PyYAML: 6.0.1
         PyZMQ: 23.2.0
        relenv: 0.16.0
         smmap: Not Installed
       timelib: 0.2.4
       Tornado: 4.5.3
           ZMQ: 4.3.4

System Versions:
          dist: amzn 2
        locale: utf-8
       machine: x86_64
       release: 5.10.149-133.644.amzn2.x86_64
        system: Linux
       version: Amazon Linux 2

Additional details
S3 credentials are obtained through the instances IAM role.

I was able to fix the issue with the following patch

diff --git a/salt/fileserver/s3fs.py b/salt/fileserver/s3fs.py
index d3c3d9cd78..cde804beb2 100644
--- a/salt/fileserver/s3fs.py
+++ b/salt/fileserver/s3fs.py
@@ -590,8 +590,9 @@ def _prune_deleted_files(metadata):
                 if os.path.exists(root):
                     roots.add(root)

-            for meta in env_data:
-                cached_files.add(meta["Key"])
+                for meta in env_data:
+                    for obj in meta[bucket]:
+                        cached_files.add(obj["Key"])

     if log.isEnabledFor(logging.DEBUG):
         import pprint

Start in salt/fileserver/s3fs.py at _prune_deleted_files and inspect the traceback alongside the reporter's proposed patch. Reproduce the s3fs cache update with the shown configuration and verify that the cache refresh completes without the KeyError.