Add migration from GitBucket (#16767)

This PR adds [GitBucket](https://gitbucket.github.io/) as migration source.

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There is no public usable instance so no integration tests added.

modules/convert/utils.go

@@ -35,6 +35,8 @@ func ToGitServiceType(value string) structs.GitServiceType {
 		return structs.GogsService
 	case "onedev":
 		return structs.OneDevService
+	case "gitbucket":
+		return structs.GitBucketService
 	default:
 		return structs.PlainGitService
 	}