Remove mention of GITGUARDIAN_API_URL

[Walz]

• Make use of GITGUARDIAN_INSTANCE in the unittest
• Remove mention of GITGUARDIAN_API_URL in documents

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (777fb39) 92.10% compared to head (0523226) 92.10%. Report is 2 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #813 +/- ## ======================================= Coverage 92.10% 92.10% ======================================= Files 159 159 Lines 6663 6663 ======================================= Hits 6137 6137 Misses 526 526 ``` | [Flag](https://app.codecov.io/gh/GitGuardian/ggshield/pull/813/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/GitGuardian/ggshield/pull/813/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | `92.10% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian#carryforward-flags-in-the-pull-request-comment) to find out more.

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