Unable to use firestore.SERVER_TIMESTAMP

## Environment

* Operating System version: Windows 10 Pro 10.0.19045 Build 19045
* Firebase SDK version: 6.1.0
* Firebase Product: Firestore (auth, database, storage, etc)
* Python version: 3.10.8
* Pip version: 22.3.1

#### Steps to reproduce:

Trying to set the timestamp when creating or updating a Firestore with ```firestore.SERVER_TIMESTAMP``` but go error message ```TypeError: Not a cmessage```.

#### Relevant Code:
```
from firebase_admin import credentials, firestore

cred = credentials.Certificate(...)
firebase_admin.initialize_app(cred)

client = firestore.client()
collection = client.collection('test')
u = uuid.uuid4()
collection.document(str(u)).set({
'id': str(u),
'timestamp': firestore.SERVER_TIMESTAMP
})
```

Full error message:
```
"/opt/homebrew/Caskroom/miniforge/base/envs/apa/lib/python3.10/asyncio/runners.py", line 44, in run
return loop.run_until_complete(main)
File "/opt/homebrew/Caskroom/miniforge/base/envs/apa/lib/python3.10/asyncio/base_events.py", line 649, in run_until_complete
return future.result()
File "/Users/hiro/Documents/apa/src/backend/pipeline/pipeline.py", line 65, in run
self.collection.document(str(self.uuid)).update({
File "/opt/homebrew/Caskroom/miniforge/base/envs/apa/lib/python3.10/site-packages/google/cloud/firestore_v1/document.py", line 324, in update
batch, kwargs = self._prep_update(field_updates, option, retry, timeout)
File "/opt/homebrew/Caskroom/miniforge/base/envs/apa/lib/python3.10/site-packages/google/cloud/firestore_v1/base_document.py", line 239, in _prep_update
batch.update(self, field_updates, option=option)
File "/opt/homebrew/Caskroom/miniforge/base/envs/apa/lib/python3.10/site-packages/google/cloud/firestore_v1/base_batch.py", line 141, in update
write_pbs = _helpers.pbs_for_update(
File "/opt/homebrew/Caskroom/miniforge/base/envs/apa/lib/python3.10/site-packages/google/cloud/firestore_v1/_helpers.py", line 941, in pbs_for_update
update_pb.update_transforms.extend(field_transform_pbs)
TypeError: Not a cmessage
```

Pip list environment
```
absl-py 1.4.0
anyio 3.6.2
appnope 0.1.3
argon2-cffi 21.3.0
argon2-cffi-bindings 21.2.0
arrow 1.2.3
astroid 2.14.2
asttokens 2.2.1
astunparse 1.6.3
attrs 22.2.0
backcall 0.2.0
beautifulsoup4 4.11.1
bleach 5.0.1
CacheControl 0.12.11
cached-property 1.5.2
cachetools 5.2.1
certifi 2022.12.7
cffi 1.15.1
charset-normalizer 3.0.1
comm 0.1.2
contourpy 1.0.7
cryptography 39.0.2
cycler 0.11.0
Cython 0.29.33
debugpy 1.6.5
decorator 5.1.1
defusedxml 0.7.1
dill 0.3.6
entrypoints 0.4
exceptiongroup 1.1.0
executing 1.2.0
fastjsonschema 2.16.2
firebase-admin 6.1.0
flatbuffers 23.1.4
fonttools 4.38.0
fqdn 1.5.1
gast 0.4.0
google-api-core 2.11.0
google-api-python-client 2.80.0
google-auth 2.16.0
google-auth-httplib2 0.1.0
google-auth-oauthlib 0.4.6
google-cloud-core 2.3.2
google-cloud-firestore 2.10.0
google-cloud-storage 2.7.0
google-crc32c 1.5.0
google-pasta 0.2.0
google-resumable-media 2.4.1
googleapis-common-protos 1.58.0
grpcio 1.51.3
grpcio-status 1.51.3
h5py 3.6.0
httplib2 0.21.0
idna 3.4
iniconfig 2.0.0
ipykernel 6.20.2
ipython 8.8.0
ipython-genutils 0.2.0
isoduration 20.11.0
isort 5.12.0
jedi 0.18.2
Jinja2 3.1.2
jsonpointer 2.3
jsonschema 4.17.3
jupyter_client 7.4.9
jupyter_core 5.1.3
jupyter-events 0.6.3
jupyter_server 2.1.0
jupyter_server_terminals 0.4.4
jupyterlab-pygments 0.2.2
keras 2.11.0
kiwisolver 1.4.4
lazy-object-proxy 1.9.0
libclang 15.0.6.1
Markdown 3.4.1
MarkupSafe 2.1.2
matplotlib 3.6.2
matplotlib-inline 0.1.6
mccabe 0.7.0
mistune 2.0.4
msgpack 1.0.4
mypy 1.0.1
mypy-extensions 1.0.0
nbclassic 0.4.8
nbclient 0.7.2
nbconvert 7.2.8
nbformat 5.7.3
nest-asyncio 1.5.6
notebook 6.5.2
notebook_shim 0.2.2
numpy 1.24.1
oauthlib 3.2.2
opencv-python 4.7.0.68
opt-einsum 3.3.0
packaging 23.0
pandas 1.5.2
pandocfilters 1.5.0
parso 0.8.3
pexpect 4.8.0
pickleshare 0.7.5
Pillow 9.4.0
pip 22.3.1
platformdirs 2.6.2
pluggy 1.0.0
prometheus-client 0.15.0
prompt-toolkit 3.0.36
proto-plus 1.22.2
protobuf 4.22.0
psutil 5.9.4
ptyprocess 0.7.0
pure-eval 0.2.2
pyasn1 0.4.8
pyasn1-modules 0.2.8
pycparser 2.21
pydash 6.0.2
Pygments 2.14.0
PyJWT 2.6.0
pylint 2.16.2
pyobjc-core 9.0.1
pyobjc-framework-Cocoa 9.0.1
pyparsing 3.0.9
pyrsistent 0.19.3
pytest 7.2.1
python-dateutil 2.8.2
python-json-logger 2.0.4
pytz 2022.7.1
PyYAML 6.0
pyzmq 25.0.0
requests 2.28.2
requests-oauthlib 1.3.1
rfc3339-validator 0.1.4
rfc3986-validator 0.1.1
rsa 4.9
screeninfo 0.8.1
Send2Trash 1.8.0
setuptools 66.1.0
six 1.16.0
sniffio 1.3.0
soupsieve 2.3.2.post1
stack-data 0.6.2
tensorboard 2.11.2
tensorboard-data-server 0.6.1
tensorboard-plugin-wit 1.8.1
tensorflow-estimator 2.11.0
tensorflow-macos 2.11.0
termcolor 2.2.0
terminado 0.17.1
tinycss2 1.2.1
tomli 2.0.1
tomlkit 0.11.6
tornado 6.2
traitlets 5.8.1
typing_extensions 4.4.0
uri-template 1.2.0
uritemplate 4.1.1
urllib3 1.26.14
wcwidth 0.2.6
webcolors 1.12
webencodings 0.5.1
websocket-client 1.4.2
Werkzeug 2.2.2
wheel 0.38.4
wrapt 1.14.1
```