Modernize postcode service integrations

[Jord-JD]

Summary

• replace the legacy Postcode Anywhere SOAP integration with Loqate’s current HTTPS JSON Find/Retrieve API
• add safe postcode normalization and reject invalid service input before network requests
• allow custom Guzzle clients and make provider response mapping resilient to optional fields
• test provider adapters deterministically on PHP 7.1 and PHP 8.5
• replace obsolete Travis/Coveralls configuration with GitHub Actions
• document current providers, normalization, HTTP customization, errors, and compatibility

Compatibility

• retains the PHP 7.1 minimum
• retains Guzzle 6 support while selecting security-fixed 6.x/7.x versions
• supports current PHPUnit releases where the running PHP version allows them

Validation

• composer.json parsed as valid JSON
• git diff --check
• GitHub Actions matrix is the executable PHP/Composer gate because PHP and Composer are not installed in the local environment