fix(mercure)!: fix Mercure transport configuration

[7-zete-7]

Fixes #870

The MERCURE_TRANSPORT_URL env has been removed and replaced by the mercure.transport Caddy directive.

BREAKING CHANGE: The MERCURE_TRANSPORT_URL env is no longer available.

Update guide

Instead of using the MERCURE_TRANSPORT_URL env, use the MERCURE_BOLT_PATH and MERCURE_BOLT_EXTRA_DIRECTIVES envs.

• Old usage:

  MERCURE_TRANSPORT_URL=bolt:///var/mercure.db

  New usage:

  MERCURE_BOLT_PATH=/var/mercure.db

• Old usage:

  MERCURE_TRANSPORT_URL=bolt:///data/mercure.db?cleanup_frequency=0.2

  New usage:

  MERCURE_BOLT_PATH=/data/mercure.db
  MERCURE_BOLT_EXTRA_DIRECTIVES=cleanup_frequency 0.2

See https://github.com/dunglas/mercure/blob/c7cf56e647e87aa814331e5ebf6672cae1ed2a54/docs/UPGRADE.md#017