Add Product Managers to FCL channels

Why is this change being made?

From an internal Slack thread, @david shared this summary of a conversation with @clefelhocz1 and @sloyd:

we agreed that better visibility for Product Leadership into FCLs would be beneficial for transparency and collaboration across departments. I will open an MR to include notifying the corresponding Product Section Leader via the public slack channel when it is created. The goal with this update is transparency so the Product Section Leader can support Infrastructure and Development teams.

This MR adds the PM, the stage Product leader, and the section Product leader to the created FCL channels.

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