Adds warnings and workarounds for Brotli performance in koa-compress

[albatrocity]

Description of what you did:

Added notes about gzip performance via koa-compress' default Brotli settings and a recommendation to disable Brotli on Heroku. These changes were prompted by the discussion in #7439 (comment) after experiencing extremely long response times on Heroku for the Admin panel javascript assets.

[codecov]

Codecov Report

Merging #8261 into documentation will increase coverage by 13.90%.
The diff coverage is n/a.

@@                Coverage Diff                 @@
##           documentation    #8261       +/-   ##
==================================================
+ Coverage          19.14%   33.04%   +13.90%     
==================================================
  Files                855     1219      +364     
  Lines              11933    13573     +1640     
  Branches            1898     1348      -550     
==================================================
+ Hits                2284     4485     +2201     
- Misses              8100     8206      +106     
+ Partials            1549      882      -667     

Flag	Coverage Δ
#front	24.70% <ø> (+10.21%)	⬆️
#unit	54.13% <ø> (+14.19%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/strapi-plugin-email/admin/src/pluginId.js	0.00% <0.00%> (-100.00%)	⬇️
...api-plugin-users-permissions/admin/src/pluginId.js	0.00% <0.00%> (-100.00%)	⬇️
...n/src/containers/NotificationProvider/constants.js	0.00% <0.00%> (-100.00%)	⬇️
...n/admin/src/containers/LanguageProvider/reducer.js	0.00% <0.00%> (-61.54%)	⬇️
packages/strapi/lib/services/metrics/index.js	65.00% <0.00%> (-35.00%)	⬇️
...admin/admin/src/components/ContainerFluid/index.js	75.00% <0.00%> (-25.00%)	⬇️
...trapi-admin/admin/src/containers/LeftMenu/index.js	42.30% <0.00%> (-24.36%)	⬇️
...admin/src/containers/LanguageProvider/selectors.js	50.00% <0.00%> (-21.43%)	⬇️
...in/admin/src/containers/Webhooks/EditView/index.js	18.57% <0.00%> (-19.83%)	⬇️
...-admin/admin/src/containers/HomePage/components.js	40.00% <0.00%> (-17.15%)	⬇️
... and 1303 more

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[derrickmehaffy]

LGTM