fix(core): fix build errors in fsharp init-template

[mikhailshilkov]

Fixes #3228.

Improved the F# template to match the C# one:

• Add a reference to Amazon.CDK.AWS.SNS.Subscriptions
• Use topic.AddSubscription instead of SubscribeQueue
• Use float instead of int for Duration constructor
• Make the code look a bit more like idiomatic F#, e.g. get rid of unneeded mutable

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