Unify the SSO config for both GitHub and GitHub Enterprise

[nghialv]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

Unify the SSO config for both GitHub and GitHub Enterprise. GITHUB_ENTERPRISE provider name was deprecated.

[pipecd-bot]

Code coverage for golang is 33.21%. This pull request increases coverage by 0.04%.

File	Function	Base	Head	Diff
pkg/oauth/github/github.go	NewOAuthClient	0.00%	0.00%	+0.00%
pkg/app/api/authhandler/callback.go	getUser	0.00%	0.00%	+0.00%
pkg/model/project.go	ProjectSSOConfig.GenerateAuthCodeURL	0.00%	0.00%	+0.00%
pkg/oauth/github/github.go	newGHEOAuthClient	0.00%	--	+-0.00%

[pipecd-bot]

Code coverage for golang is 33.28%. This pull request increases coverage by 0.01%.

File	Function	Base	Head	Diff
pkg/app/api/authhandler/callback.go	getUser	0.00%	0.00%	+0.00%
pkg/model/project.go	ProjectSSOConfig.GenerateAuthCodeURL	0.00%	0.00%	+0.00%
pkg/oauth/github/github.go	NewOAuthClient	0.00%	0.00%	+0.00%
pkg/oauth/github/github.go	newGHEOAuthClient	0.00%	--	+-0.00%

[pipecd-bot]

Code coverage for golang is 33.28%. This pull request increases coverage by 0.04%.

File	Function	Base	Head	Diff
pkg/app/api/authhandler/callback.go	getUser	0.00%	0.00%	+0.00%
pkg/model/project.go	ProjectSSOConfig.GenerateAuthCodeURL	0.00%	0.00%	+0.00%
pkg/oauth/github/github.go	NewOAuthClient	0.00%	0.00%	+0.00%
pkg/oauth/github/github.go	newGHEOAuthClient	0.00%	--	+-0.00%

[gkuga]

/approve

[nghialv]

/approve

[pipecd-bot]

This pull request is APPROVED by nghialv.

Approvers can cancel the approval by writing /approve cancel in a comment. Any additional commits also will change this pull request to be not-approved.