fix: remove old cache on serial upgrade in repareCache

[deepin-wm]

When the serial of a dconfig configuration is upgraded, the old cache values should be discarded. Currently repareCache() does not handle serial upgrades, causing the stale cache to persist and checkSerial() to fail.

Changes:

1. repareCache(): Added serial upgrade handling — when newMeta->serial(key) > oldMeta->serial(key), use cache->remove(key) to remove the old cache value
2. Added comment: // Serial升级时移除旧缓存值，serial升级意味着旧值必须废弃
3. Added debug log for serial upgrade cache removal

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[deepin-ci-robot]

deepin pr auto review

★ 总体评分：100分

■ 【总体评价】

代码精准修复了Serial升级时旧缓存未清除的缺陷，逻辑严密且无安全风险
语法逻辑完全正确，代码质量良好，性能无损耗，满足满分要求

■ 【详细分析】

• 1.语法逻辑（完全正确）✓

新增代码在 dconfigresource.cpp 的 DSGConfigResource::repareCache 函数末尾，通过遍历 intersectKeys 并比较 newMeta->serial(key) 与 oldMeta->serial(key) 的大小关系来决定是否调用 cache->remove(key)，逻辑严密，符合C++标准语法及Qt编程规范。

• 2.代码质量（良好）✓

新增代码包含清晰的注释说明修复意图，日志输出使用 qDebug 并携带了 resource、uid、key 等关键上下文信息，便于问题追踪与排查，且与原有代码风格保持高度一致。

• 3.代码性能（无性能问题）✓

新增的 for 循环仅针对配置交集键进行遍历，在配置管理场景中交集键数量通常极少，serial() 比较与 remove() 操作的时间复杂度可视为常数级，不会引入性能瓶颈。

• 4.代码安全（存在0个安全漏洞）✓

漏洞对比统计：新增漏洞 0 个，减少漏洞 0 个，持平 0 个
代码未引入任何新的安全风险，日志打印的 m_key 和 key 均为系统预定义的配置元数据键名，非外部不可信输入，不存在注入或信息泄露风险；指针使用均在安全上下文中。

■ 【改进建议代码示例】

// 当前代码已足够优秀，无需额外修改。以下为保留原始逻辑的完整上下文示意：
void DSGConfigResource::repareCache(DConfigCache *cache, DConfigMeta *oldMeta, DConfigMeta *newMeta)
{
    // ... (前置的 intersectKeys 处理逻辑) ...

    // Serial升级时移除旧缓存值，serial升级意味着旧值必须废弃
    for (const auto &key : intersectKeys) {
        if (newMeta->serial(key) > oldMeta->serial(key)) {
            cache->remove(key);
            qDebug(cfLog, "Cache removed because of serial upgraded, resource:%s, uid:%d, key:%s.",
                   qPrintable(m_key), cache->uid(), qPrintable(key));
        }
    }
}