refactor: Refactor org_custom_roles.go into multiple files

[felixlut]

Continuation of #3281 (comment)

The /organizations/ORGANIZATION_ID/custom_roles endpoint has been deprecated in favor of /orgs/ORG/custom-repository-roles. So the orgs_custom_roles.go file should be refactored. The file also deals with Organization roles, which is a separate kind of resource to the Organization Repository Roles. For this reason this PR aims to separate the file into two:

• orgs_custom_repository_roles.go - API docs - /orgs/ORG/custom-repository-roles
• orgs_organization_roles.go - API docs - /orgs/ORG/organization-roles

[gmlewis]

Thank you, @felixlut !
Any time a new file is created, we bump the copyright year to the current year, but if a file is renamed or otherwise edited, we leave the copyright year alone...
So just a couple minor nits, please, then we should be ready for a second LGTM+Approval from any other contributor to this repo before merging.

[codecov]

Codecov Report

Attention: Patch coverage is 95.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.97%. Comparing base (2b8c7fa) to head (d590920).
Report is 127 commits behind head on master.

Files with missing lines	Patch %	Lines
github/orgs_custom_repository_roles.go	95.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@ Coverage Diff @@## master #3291 +/- ##
==========================================
- Coverage 97.72% 92.97% -4.75% 
==========================================
Files 153 172 +19 Lines 13390 11715 -1675 ==========================================
- Hits 13085 10892 -2193 - Misses 215 729 +514 - Partials 90 94 +4 

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[gmlewis]

Thank you, @felixlut !
LGTM.

Awaiting second LGTM+Approval from any other contributor to this repo before merging.

[tomfeigin]

LGTM

[gmlewis]

Thank you, @tomfeigin !
Merging.