Increase the stale interval (or threshold) from 30 to 180 days since last activity in PR (RFC book)

[anaelleltd]

Some of the info displayed in the RFC book is inaccurate.
For example, there are a lot of PRs that have been filed in the "stale" category, though they are less than 3 months old.
Also, draft PRs are tagged as "stale", even though they are ongoing.

@paritytech/opstooling

[anaelleltd]

Specifically, the following RFCs should not be in the Stale category:

Edit: It would be good to increase the stale interval (or threshold) from 30 to 180 days since last activity in PR. This would re-render the mdbook and re-group the RFCs accordingly to the new rule.

Thanks!