fleet: require a minimum version of the peer at the handshake [CI]

MoShiro__MinAgentVersion makes the backend refuse agents below it at hello (fleet_version_required); fleet.min_server_version or MOSHIRO_MIN_SERVER_VERSION makes the agent drop a backend below it. Either side waits five seconds and shakes hands again, because the other one may be updating.

backend/src/MoShiro.Backend.Application/BackendOptions.cs

@@ -60,6 +60,9 @@ public sealed class BackendOptions
 
     public int LoginAttemptsPerMinute { get; set; } = 10;
 
+    /// <summary>Agent 至少要是哪个版本才能连上（如 <c>0.4.2-dev.80</c>）；空 = 不要求。低于它的 Agent 握手时被拒绝，5 秒后重试（它可能正在更新）。环境变量 <c>MoShiro__MinAgentVersion</c>。</summary>
+    public string? MinAgentVersion { get; set; }
+
     public int RawRetentionDays { get; set; } = 30;
 
     public int TestRawRetentionDays { get; set; } = 90;