Update python specifier to follow best practices, be explicit and remove warning

At the moment the python version is specified as follows:

requires-python = "~=3.11"

Even though quite compact it suffers from implicitness that can lead to bugs later one. The warning from uv explains it better:

warning: The requires-python specifier (~=3.11) in fastapi-boilerplate uses the tilde specifier (~=) without a patch version. This will be interpreted as >=3.11, <4. Did you mean ~=3.11.0 to constrain the version as >=3.11.0, <3.12? We recommend only using the tilde specifier with a patch version to avoid ambiguity.

Instead is better to be explicit. This PR changes the version specific to be explicit which also removes the waning from uv.