[BUGFIX] Re-enable Guzzle versions > 6.5.0

Guzzle 6.5.0 introduced an issue with umlaut domains
which was fixed in Guzzle 6.5.1 and Guzzle 6.5.2
respectively.

The conflict section in composer.json is now
updated, in order to allow composer-based TYPO3
installations to
a) allow TYPO3 updates to 10.3.x/9.5.14/8.7.31 even
if guzzle has been locked in composer.lock to 6.5.2 already
b) allow to use the latest Guzzle fixes again

Used composer command:
  composer update --lock

See https://github.com/guzzle/guzzle/issues/2448 which
was integrated in Guzzle 6.5.1 (which was released AFTER
our core release in December 2019)

Resolves: #90634
Releases: master, 9.5, 8.7
Change-Id: If6c2f3ef6529109148569f1af9472b0bcb44c9b9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63548


Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>