fix: close response body and fix memory leak

What does this MR do?

fixes a few memory leaks I bumped into while reviewing the code:

• makeRequest: this was documented so it's an easy fix, see https://gitlab.com/gitlab-org/gitlab-pages/-/blob/master/internal/handlers/handlers.go#L65-L66
• fetchAccessToken: this was more tricky, if StatusCode is not 200 (OK) the method returns early and the body is never closed
• checkAuthentication: the statement is missing and the body is never closed*

*technically, the body is closed inside checkResponseForInvalidToken which is called here. However the logic is run only if status code is StatusUnauthorized.

TODO

• I added the Changelog trailer (e.g. Changelog: feature) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines

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