fix: Remove non required fields (#31)

Co-authored-by: ProcessOut Fountain <internal@processout.com>

feat!: remove deprecated functionality and drop PHP unsupported versi…

…on compatibility

BREAKING CHANGE: 

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

> [!CAUTION]
> BREAKING CHANGE.
> The SDK supports PHP version 8.0 and newer. Versions lower than 8.0 are not supported any longer!

## Address deprecated functionality warnings:

<details><summary>

### 1. Set jsonSerialize() return type explicitly

</summary>

* Since [8.2](https://github.com/php/php-src/blob/PHP-8.1.0/ext/json/json.stub.php#L18) - `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
```

</details>


<details><summary>

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

</summary>

* Since [8.0](https://php.watch/versions/8.0/resource-CurlHandle) - `curl_close()` has no effect and is deprecated since [8.5](https://php.watch/versions/8.5/curl_close-curl_share_close-deprecated).

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

</details>

<details><summary>

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

</summary>

* Since [8.0](https://php.watch/versions/8.0/null-safe-operator) - 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
```

</details>

## Remove non-existent endpoints and related resources
* /activities
* /coupons
* /customers/:id/subscriptions
* /plans
* /subscriptions
* /supervised-projects

Merge pull request #28 from processout/pathikrit-modak-cko-patch-1

chore: update go-semantic-release

feat: add customer and invoice reference_id; add balances customer ac…

…tion (#27)

Co-authored-by: ProcessOut Fountain <internal@processout.com>

Merge pull request #26 from processout/feat/update-library-1744634595

feat: Update library 1744634595

Merge pull request #26 from processout/feat/update-library-1744634595

feat: Update library 1744634595

feat: add-new-operation-error-message-field (#25)

Co-authored-by: ProcessOut Fountain <internal@processout.com>

feat: Add custom action structure to the create token flow (#23)

Co-authored-by: ProcessOut Fountain <internal@processout.com>

BREAKING CHANGE: breaking: change response types for invoices-authori…

…ze, invoices-capture, invoices-native-payment; breaking: create/update card; add invoice expiry; add invoice qr_code; add invoice delete; add project public metadata; add hosted payment page support (#22)

Co-authored-by: ProcessOut Fountain <internal@processout.com>

feat: add auto_capture_at field within the invoice object (#20)

Co-authored-by: ProcessOut Fountain <internal@processout.com>