feat: bumped php-govuk-account for did document support

[jerotire]

Description

Bumps dvsa/php-govuk-account to 3.0.0 which deprecates defining the Core Identity JWT public key statically, and instead introduces a DID Document to obtain it's public key.

Related config changes implemented too for config.global.php and local.php.dist.

Related Terraform PR for environment configuration:

• https://github.com/dvsa/vol-terraform/pull/353 (non-prod environments)
• https://github.com/dvsa/vol-terraform/pull/355 (prod environments)

Related issue: https://dvsa.atlassian.net/browse/VOL-5837

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