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/agent/src/MoShiro.Agent/AgentApp.cs

@@ -353,6 +353,7 @@ public sealed class AgentApp : IDisposable, IAsyncDisposable
             ServerVerifier = _trust,
             NodeId = _identity.NodeId,
             NodeVersion = Version,
+            MinServerVersion = () => Environment.GetEnvironmentVariable("MOSHIRO_MIN_SERVER_VERSION") is { Length: > 0 } required ? required : _config.Fleet.MinServerVersion,
             Capabilities = [MoShiroCapabilities.Base, MoShiroCapabilities.KindsPrefix + string.Join(',', _listening ? BlockCatalog.AgentKinds : BlockCatalog.TargetKinds)],
             HelloMeta = HelloMeta,
             HeartbeatExtra = HeartbeatExtra,