StagingMode: Change default to non-readonly (microsoft#25565)

Defaulting to readonly has proved challenging for prototyping, as it
requires quite a bit more setup to use the feature. Given this is an
alpha feature, which isn't currently in use, changing the default should
present no issues, and should make further feature exploration easier.

Author: anthony-murphy

packages/runtime/container-runtime/src/dataStoreContext.ts

@@ -282,7 +282,7 @@ export abstract class FluidDataStoreContext
 
 	private isStagingMode: boolean = false;
 	public isReadOnly = (): boolean =>
-		(this.isStagingMode && this.channel?.policies?.readonlyInStagingMode !== false) ||
+		(this.isStagingMode && this.channel?.policies?.readonlyInStagingMode === true) ||
 		this.parentContext.isReadOnly();
 
 	public get connected(): boolean {