Update dependency sinatra to v4

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
sinatra (source, changelog)	"~> 3.1" -> "~> 4.0"

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Release Notes

sinatra/sinatra (sinatra)

v4.0.0

Compare Source

• New: Add support for Rack 3 (#​1857)

  • Note: you may want to read the Rack 3 Upgrade Guide

• Require Ruby 2.7.8 as minimum Ruby version (#​1993)

• Breaking change: Drop support for Rack 2 (#​1857)

  • Note: when using Sinatra to start the web server, you now need the rackup gem installed

• Breaking change: Remove the IndifferentHash initializer (#​1982)

• Breaking change: Disable session_hijacking protection by default (#​1984)

• Breaking change: Remove Rack::Protection::EncryptedCookie (#​1989)

  • Note: cookies are still encrypted (by [Rack::Session::Cookie][Rack::Session::Cookie])

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[ThomasKoppensteiner]

Fix the secret first:

ArgumentError:
        invalid secret: 25, must be >=64