Remove support for Python 3.8

[julian-risch]

We switched from Python 3.8 to 3.9 for the CI with PR #8492 and Python 3.8 reached end of life in October 2024.
We should remove/update the lines of code that are only needed for Python 3.8 support, in particular:

• haystack/haystack/core/component/types.py

  Line 8 in 7d51793

  from typing_extensions import Annotated, TypeAlias # Python 3.8 compatibility

• haystack/pyproject.toml

  Line 11 in 7d51793

  requires-python = ">=3.8,<3.13"

• haystack/pyproject.toml

  Line 37 in 7d51793

  "Programming Language :: Python :: 3.8",

• haystack/haystack/core/component/component.py

  Line 160 in 7d51793

  # NOTE: This check can be removed when we drop Python 3.8 support.