chore: drop support for 3.8

[masci]

Description

Python 3.8 was EOL'ed last October and won't receive further upgrades. We already updated our CI and made 3.9 the new base reference, and this change reflects that at the package level.

Also with this PR:

• Updated the lockfiles
• Fixed a poetry install problem with numpy<2
• Added eval-type-backport as dependency for 3.9, so we can use from __future__ import annotations alongside Pydantic 2 and start modernizing our codebase

Related to #16962 this will make things easier there.

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core package)

• Yes
• No

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Your pull-request will likely not be merged unless it is covered by some form of impactful unit testing.

• I added new unit tests to cover this change
• I believe this change is already covered by existing unit tests

[logan-markewich]

hmmm can you try running poetry lock again?

[logan-markewich]

with this change, we could also set numpy = "*" in core if we wanted to