docs: Update the manifest reference to the current

[mastier]

The current manifest file reference is not up-to-date

While deploying with current openstack snap 2024.1/stable rev922
I got warning:

           WARNING  Manifest uses deprecated 'external_network' field, please update to 'external-networks'                                                                                 configure.py:398

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Reported from: https://canonical-openstack.readthedocs-hosted.com/en/latest/reference/manifest-file-reference/

[gboutry]

Hello @mastier, thanks for reporting, are you talking about the field that is tagged:
# External networking (deprecated) ?

canonical-openstack-docs/reference/manifest-file-reference.rst

Line 237 in 8b2c212

external_network:

[mastier]

Hello @gboutry , yes exactly :-)
I will try my best to help you find the gaps.