Fix Github constant autoload in dev

What does this MR do?

This is basically the same fix applied to the Jira constant (!62437 (merged))

In !62676 (merged) we renamed a model file from GithubService to Integrations::Github.

Subsequently, in development mode, Rails would error when we called the service files that are in the top-level ::Github namespace, rather than load them:

• app/services/github
• ee/app/services/github

Screenshots (strongly suggested)

** Before the fix (in dev):

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
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• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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