Reduce Downtime through Upgrade Orchestration [GA]

### Value Prop
Upgrading GitHub Enterprise Server now requires about 20 minutes less downtime. Starting with GHES 3.21, pre-upgrade steps that previously ran during the maintenance window can now be run beforehand, while the appliance is still serving traffic.

### Expected Outcome
Downtime during upgrades is one of the most significant friction points for GHES customers. Starting in 3.21, the pre-upgrade steps run while the appliance is still live, moving ~20 minutes of work outside the maintenance window and reducing total downtime accordingly.

路线图 issue 中未确定任何文件、测试或入口点。首先明确 GHES 3.21 的升级工作流和实现范围；完成的标准是：预升级工作在 appliance 提供流量服务的同时运行，并将维护窗口的停机时间缩短约 20 分钟。

由索引模型根据 Issue 内容生成。