feat(manager/gomod): improve commitMessageTopic for go/toolchain directive bumps

[jamietanna]

Changes

As noted in #42519, the preivous commit/PR title wasn't quite as clear
with what was actually changing.

To make this more explicit, we can override commitMessageTopic.

Context

Please select one of the following:

• This closes an existing Issue, Closes: Improve commit/PR title for updates to Go toolchain and go directives #42519
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

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Please select one option and, if yes, briefly describe how AI was used (e.g., code, tests, docs) and which tool(s) you used.

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• Yes — substantive assistance (AI-generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe):

Documentation (please check one with an [x])

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• No unit tests, but ran on a real repository, or
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The public repository: