fix: always show the GitHub endpoint selection

[carlossg]

Description

so users are aware that they must add a new endpoint in configiuration

Standardize the docs for apiUri

Submitter checklist

• Link to JIRA ticket in description, if appropriate.
• Change is code complete and matches issue description
• Automated tests have been added to exercise the changes
• Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• Run the changes and verify that the change matches the issue description
• Reviewed the code
• Verified that the appropriate tests have been written or valid explanation given

Documentation changes

• Link to jenkins.io PR, or an explanation for why no doc changes are needed

Users/aliases to notify

[carlossg]

I checked the source and app credential rendering, but not sure in what page is GitHubSCMNavigator used?

[bitwiseman]

I'm pretty sure this drop down will also include github.com if the endpoint list is empty. Or did that change at some point?

[carlossg]

it does not include github.com

[bitwiseman]

Were saying this path is deprecated anyway as of #262 .

In #254, I attempted to address a bad behavior in JobDSL caused by #262 but that lost steam.

Could you talk a bit more about how this is an improvement?

[bitwiseman]

@carlossg Ping?

[carlossg]

Today if you add a new GitHub App credential there is no option to select a GitHub enterprise installation. You must know that you need to go to settings - manage jenkins and add the endpoint.
This makes it explicit, it shows the github.com endpoint and in the help it tells you how to add a new endpoint, so it saves time to new users

[carlossg]

before:

after: