chore: update protodoc strings

[faddat]

Description

This PR updates protodoc strings to omit the claim that governance is the default signer / authority.

closes: #4668

Commit Message / Changelog Entry

chore: omit governance as the default signer in protodoc strings

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• Wrote unit and integration tests.
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[damiannolan]

Thank you sir!

[DimitrisJim]

amended the doc for SoftwareUpgrade message which was missed, lgtm