Backport of license: MPL'ify proto-public into release/1.16.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #20143 to be assessed for backporting due to the inclusion of the label backport/1.16.

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The person who merged in the original PR is:
@analogue
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

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The below text is copied from the body of the original PR.

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Description

See NET-5265.
This is an continuation of #19780 since we didn't have a proto-public/.copywrite.hcl file at that time.

• copywrite headers used to add missing MPL license headers
• BUSL -> MPL was manual since copywrite headers can't fix those (maybe one day?)

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

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Overview of commits

• 5fabd10 - ad5e95a - ed0e44c

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[github-team-consul-core-pr-approver]

Auto approved Consul Bot automated PR

[analogue]

Closing: superseded by #20156