mysql8: use protobuf

[suhailskhan]

Description

Spun off from #28321 to prevent CI from timing out.

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 15.4.1 24E263 arm64
Xcode 16.3 16E140

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

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[suhailskhan]

Before changing the protobuf3-cpp dependency to protobuf, I pushed a commit that only changed the revision number and nothing else.

CI check results after this commit:
https://github.com/macports/macports-ports/actions/runs/15121263309?pr=28485

CI check results after changing dependency from protobuf3-cpp to protobuf:
https://github.com/macports/macports-ports/actions/runs/15122682284?pr=28485