feat!: remove deprecated functionality and drop PHP unsupported version compatibility

[tomasz-pazdziurek-cko]

BREAKING CHANGE:

This PR has been automatically generated by fountain. Please review it and merge it if everything is fine.

Caution

This is a BREAKING CHANG.
The SDK supports PHP version 8.0 and newer. Versions lower than 8.0 are not supported any longer!

This change addresses a couple of deprecated functionality warnings:

1. Set jsonSerialize() return type explicitly

• Since 8.2 - JsonSerializable requires the implementations to explicitly return the returned type.

Deprecated: Return type of ProcessOut\Invoice::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

2. Remove curl_close() invocations - it's being done automatically during GC now.

• Since 8.0 - curl_close() has no effect and is deprecated since 8.5.

Deprecated: Function curl_close() is deprecated since 8.5, as it has no effect since PHP 8.0

3. Add "?" explicit nullable type operator when applicable.

• Since 8.0 - added support for "nullable" types - nullability should be explicitly expressed using the ? operator.

Deprecated: ProcessOut\Exceptions\AuthenticationException::__construct(): Implicitly marking parameter $previous as nullable is deprecated, the explicit nullable type must be used instead

4. Remove non-existent endpoints and related resources

• /activities
• /coupons
• /customers/:id/subscriptions
• /plans
• /subscriptions
• /supervised-projects

[tomasz-pazdziurek-cko]

https://www.php.net/supported-versions.php

As of 2026-01-12 09:30 UTC, "PHP: Supported Versions" look as below (expand):

PHP: Supported Versions

As we can see, version 8.0 is no longer supported, since the end of 2023.
Dropping support for no longer supported versions of the language seems reasonable from our point of view and lets us keep our SDKs up-to-date. We do not have plans or resources to keep support for outdated versions of various languages as we would have to be committed to applying patches to overcome potential problems occurring on these outdated versions of the language itself (e.g. potential security vulnerability).

If our SDK works on version lower than specified it's purely coincidental and may stop working in any moment.
We tested this SDK version against unsupported PHP versions 8.0.30 and 8.1.34, but this compatibility shouldn't be taken as granted and we advice to use the SDK with the supported PHP versions only (8.2+ as of today).

[lukasz-k-bieszczad-cko]

lgtm :D