Revert LinkedIn OAuth2

[yuekui]

The previous PR #950 mistakenly replaced the OAuth2 endpoint with the OpenID Connect one. Users who use OpenID Connect should use LinkedinOpenIdConnect. For existing LinkedIn OAuth2 users like myself, LinkedinOAuth2 should still work.

Just to clarify: LinkedIn has NOT removed the r_liteprofile scope for OAuth2 users — it's still functioning properly. If anyone is having issues with scopes, please check the “Products” tab in your LinkedIn App Setup page.

For example, here we have both OAuth2 and OpenID Connect enabled:

[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.88%. Comparing base (345ad5f) to head (03c7bc9).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
social_core/backends/linkedin.py	76.92%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1147      +/-   ##
==========================================
+ Coverage   77.87%   77.88%   +0.01%     
==========================================
  Files         353      353              
  Lines       10794    10799       +5     
  Branches      471      471              
==========================================
+ Hits         8406     8411       +5     
  Misses       2222     2222              
  Partials      166      166              

Flag	Coverage Δ
unittests	77.88% <78.57%> (+0.01%)	⬆️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[nijel]

Merged, thanks for your contribution!

[bogdandm]

Hello, is it possible to create hotfix release with this fix? LinkedinOAuth2 does not work for our service and we have to copy-paste whole LinkedinOAuth2 from previous version as it critical to our users to be able to login into their accounts

[nijel]

I wanted to merge #1183 before the release, but it is not yet ready...