[chore] use string builder to appease linter

[codeboten]

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https://github.com/open-telemetry/opentelemetry-collector/actions/runs/19068015057/job/54463435271?pr=14121

go tool -modfile /home/runner/work/opentelemetry-collector/opentelemetry-collector/internal/tools/go.mod golangci-lint run
Error: cmd/mdatagen/internal/command.go:305:7: concat-loop: string concatenation in a loop (perfsprint)
						result += caser(part)
						^