fix: loosen gevent pin to unblock downstream consumers on gevent 25.x

[craig8]

Problem

The current gevent pin >=24.2.1,<24.3 prevents downstream packages that require
gevent>=25.x from installing alongside volttron-core. pip's dependency resolver
raises ResolutionImpossible and the install fails entirely.

Fix

Remove the upper bound: gevent = ">=24.2.1"

gevent follows a date-based versioning scheme. There is no known breaking change in
the gevent 25.x series that would affect volttron-core. The <24.3 upper bound
appears to have been added as a precaution but is now blocking legitimate use cases.

Impact

• Any project depending on both volttron-core and gevent>=25.x is currently
  uninstallable via pip
• This is a blocking issue for downstream consumers (e.g. AEMS FastAPI platform
  with gevent~=25.9.1)

Testing

The existing test suite should pass unchanged — no volttron-core code depends on
gevent 24.x-specific behavior.