Update security manager operations to use organization roles REST APIs

[just-joshing]

The organization security manager role REST APIs are being sunset (https://gh.io/security-managers-rest-api-sunset) in favor of the organization roles REST APIs.

Instead of introducing a breaking change by removing the operations from go-github, this PR updates the operations to use the organization roles REST APIs so consumers (such as Terraform) can continue to manage organization security manager teams without needing to change code.

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[codecov]

Codecov Report

Attention: Patch coverage is 82.14286% with 5 lines in your changes missing coverage. Please review.

Project coverage is 92.19%. Comparing base (2b8c7fa) to head (fc83925).
Report is 212 commits behind head on master.

Files with missing lines	Patch %	Lines
github/orgs_security_managers.go	82.14%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3418      +/-   ##
==========================================
- Coverage   97.72%   92.19%   -5.53%     
==========================================
  Files         153      173      +20     
  Lines       13390    14949    +1559     
==========================================
+ Hits        13085    13782     +697     
- Misses        215     1072     +857     
- Partials       90       95       +5     

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[just-joshing]

#3421