v6.0.0

BREAKING CHANGES:

• Configuration parameters auth_region and auth_url are made mandatory
  for the provider configuration (#313)
• Renamed environment variables for resources:
  • SEL_PROJECT_ID -> INFRA_PROJECT_ID
  • SEL_REGION -> INFRA_REGION
• Removed deprecated resources (#313):
  • selectel_vpc_role_v2
  • selectel_vpc_token_v2
  • selectel_vpc_user_v2
  • selectel_vpc_vrrp_subnet_v2
  • selectel_vpc_crossregion_subnet_v2

IMPROVEMENTS:

• Added migration guide to upgrade to new major v6 version (#313)
• Updated docs (#313)