[WIP] Update Guzzle to 6.3

[phil-davis]

This is an interim rebase in progress...

[codecov]

Codecov Report

Merging #32525 into master will decrease coverage by 0.25%.
The diff coverage is 50%.

@@             Coverage Diff              @@
##             master   #32525      +/-   ##
============================================
- Coverage     64.12%   63.86%   -0.26%     
- Complexity    18643    18645       +2     
============================================
  Files          1176     1176              
  Lines         70189    70160      -29     
  Branches       1270     1270              
============================================
- Hits          45007    44811     -196     
- Misses        24812    24979     +167     
  Partials        370      370

Flag	Coverage Δ	Complexity Δ
#javascript	52.89% <ø> (ø)	0 <ø> (ø)	⬇️
#phpunit	65.11% <50%> (-0.29%)	18645 <1> (+2)

Impacted Files	Coverage Δ	Complexity Δ
lib/private/Files/Storage/DAV.php	81.09% <ø> (ø)	0 <0> (ø)	⬇️
lib/private/Http/Client/Response.php	53.84% <100%> (ø)	6 <1> (ø)	⬇️
lib/private/Http/Client/Client.php	79.62% <44.44%> (-2.38%)	19 <0> (+2)
...ivate/Files/ObjectStore/HomeObjectStoreStorage.php	0% <0%> (-86.67%)	8% <0%> (ø)
lib/private/Files/Storage/Wrapper/Availability.php	10.38% <0%> (-40.26%)	80% <0%> (ø)
lib/private/Files/ObjectStore/NoopScanner.php	68.75% <0%> (-18.75%)	6% <0%> (ø)
lib/private/Preview/MP3.php	45.16% <0%> (-12.91%)	13% <0%> (ø)
...b/private/Files/ObjectStore/ObjectStoreStorage.php	71.37% <0%> (-12.55%)	93% <0%> (ø)
apps/files_sharing/lib/Scanner.php	85.71% <0%> (-9.53%)	7% <0%> (ø)
lib/private/App/Platform.php	21.73% <0%> (-8.7%)	9% <0%> (ø)
... and 10 more

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[phil-davis]

See #32526 for a newer version with the recent small acceptance test commits squashed.