Update upload providers README to reflect stable changes

[derrickmehaffy]

Signed-off-by: Derrick Mehaffy derrickmehaffy@gmail.com

Description of what you did:

Updated the README files of the 3 upload providers to use the ./config/plugins.js instead of extensions.

[codecov]

Codecov Report

Merging #6709 into master will increase coverage by 0.13%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master    #6709      +/-   ##
==========================================
+ Coverage   19.85%   19.98%   +0.13%     
==========================================
  Files         857      858       +1     
  Lines       12076    12097      +21     
  Branches     1963     1965       +2     
==========================================
+ Hits         2398     2418      +20     
- Misses       8093     8094       +1     
  Partials     1585     1585              

Flag	Coverage Δ
#front	14.67% <50.00%> (+<0.01%)	⬆️
#unit	41.71% <95.00%> (+0.45%)	⬆️

Impacted Files	Coverage Δ
...admin/src/containers/InstalledPluginsPage/index.js	7.14% <ø> (ø)
...kages/strapi-admin/admin/src/translations/index.js	100.00% <ø> (ø)
...-manager/admin/src/components/DynamicZone/index.js	0.00% <ø> (ø)
...in-content-manager/admin/src/translations/index.js	0.00% <ø> (ø)
...lder/admin/src/containers/FormModal/utils/forms.js	0.00% <0.00%> (ø)
...ntent-type-builder/admin/src/translations/index.js	0.00% <ø> (ø)
...ugin-documentation/admin/src/translations/index.js	0.00% <ø> (ø)
...trapi-plugin-email/admin/src/translations/index.js	0.00% <ø> (ø)
.../containers/InputModalStepper/InputModalStepper.js	0.00% <ø> (ø)
...-upload/admin/src/containers/ModalStepper/index.js	0.00% <ø> (ø)
... and 6 more

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[alexandrebodin]

LGTM

[alexandrebodin]

@derrickmehaffy can you make this PR on the documentation branch so we can deploy it directly after merging

[derrickmehaffy]

@alexandrebodin it doesn't directly change the docs, only the README.md files in the git so I think pushing it straight to master would be better as you won't need to merge in the doc branch.

I only set the doc tag as it's kinda doc related