omniauth-gitlab: Fix callback_url for relative_urls

What does this MR do and why?

• One of the breaking changes in OmniAuth 2.0+ relates to how relative URL installations are handled. See: https://github.com/omniauth/omniauth/pull/903 and https://github.com/omniauth/omniauth/wiki/Upgrading-to-2.0#relative-root-apps
• As a result, when omniauth-gitlab is used with OmniAuth 2.0+ for an app that lives at a relative URl, the #callback_url is incorrect (the relative URL is included twice).
• This is because OmniAuth is now prefixing the default Strategy#request_path and Strategy#callback_path with SCRIPT_NAME, but omniauth-gitlab is also adding script_name to callback_url. https://github.com/omniauth/omniauth-oauth2/pull/152#issuecomment-1159512977
• Similar change to omniauth-google-oauth2 was made here: https://github.com/zquestz/omniauth-google-oauth2/pull/403
• Similar change to omniauth-google-azure-activedirectory-v2 was made here: https://github.com/RIPAGlobal/omniauth-azure-activedirectory-v2/pull/6
• Fixes !97985 (comment 1101545432)

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