⬆️ Drop support for Python 3.7, require Python 3.8 or above (#10442)

* 📝 Update docs, require Python 3.8+, drop 3.7

* 🔧 Update pyproject.toml, drop support for Python 3.7, require Python 3.8+

* 👷 Update CI GitHub Actions, drop support for Python 3.7, require 3.8+

* 📝 Update docs' references to Python 3.6 and 3.7, use Python 3.8

docs/zh/docs/tutorial/security/first-steps.md

@@ -26,13 +26,13 @@
     {!> ../../../docs_src/security/tutorial001_an_py39.py!}
     ```
 
-=== "Python 3.6+"
+=== "Python 3.8+"
 
     ```Python
     {!> ../../../docs_src/security/tutorial001_an.py!}
     ```
 
-=== "Python 3.6+ non-Annotated"
+=== "Python 3.8+ non-Annotated"
 
     !!! tip
         尽可能选择使用 `Annotated` 的版本。