chore(nimbus): remove klaatu constants and pyjwt dependency

Because

* WHAT_TRAIN_IS_IT_NOW_URL was added for klaatu tasks to look up
  Firefox release versions
* pyjwt[crypto] was added for klaatu GitHub App JWT authentication
* Neither is used by any remaining code

This commit

* Removes WHAT_TRAIN_IS_IT_NOW_URL from NimbusConstants
* Removes pyjwt from pyproject.toml and regenerates poetry.lock

Fixes #15103

Author: jaredlockhart

experimenter/experimenter/experiments/constants.py

@@ -1107,7 +1107,6 @@ class FirefoxLabsGroups(models.TextChoices):
     )
     OBSERVATION = "Observation"
     ENROLLMENT = "Enrollment"
-    WHAT_TRAIN_IS_IT_NOW_URL = "https://whattrainisitnow.com/api/firefox/releases/"
 
     FIREFOX_LABS_MIN_VERSION = {
         Application.DESKTOP: Version.FIREFOX_137,