feat(ENT-923): update priority threshold in config file parser

[juicetin]

Description

• add case insensitivity for priority threshold, update to the 3 labels available in our agents
• I've marked this as major because the label change is technically a breaking change - can adjust based on comments
  • the old labels aren't explicitly used anywhere in code
  • customers may already have the old labels in their config files though - I may need to update this PR to account for that
  • though - https://docs.nullify.ai/getting-started/configuration-general priorityThreshold is not yet documented so we can still change it at will and have it technically not be a breaking change 😅

Test Plan

Please include steps to test the change. Include screenshots if applicable.

Checklist

• I have performed a self-review of my own code
• I have checked for redundant or commented out code
• I have made corresponding changes to the documentation
• I have added any appropriate tests