Update default LTI keys

[nwalters512]

The default keys used a short key length and outdated algorithm, which made them incompatible with newer LTI tools.

I also updated the instructions for generating new keys; the old instructions don't work (see #1861). I copied the call from here:

canvas-lms/gems/canvas_security/lib/canvas_security/key_storage.rb

Lines 34 to 36 in fbe666a

	def new_key
	CanvasSecurity::RSAKeyPair.new.to_jwk.to_json
	end

Closes #1861.

[CLAassistant]

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