[REF] Upgrade Guzzle to 6.5.6

[seamuslee001]

@eileenmcnaughton

[civibot]

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[totten]

It should be noted for anyone following along - this was sent in because Guzzle had a security update (as linked in the dependabot PR). However, for Civi, it appears unlikely to have an actual impact; it's only prophylactic.

• The security-issue affects cookie-handling in Guzzle, but you have to opt-in to enable cookies (eg by setting cookies and/or creating a CookieJar). Grepping civicrm-core, the opt-in only appears in a few internal test files.
• Similarly, I couldn't find any cookie opt-ins in the universe of published extensions that might rely on Civi's copy of Guzzle.
• So we haven't run this through the full rigamarole of a security release for civicrm.
• Of course, it's hard to say that it's impossible to exploit. (Maybe some private add-on uses Guzzle cookies?)
• So it's definitely good to include this update at the nearest convenience.