Update class-checkout-mutation.php

[manuelsampl]

Static Keyword missing

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What does this implement/fix? Explain your changes.

Got this Error on Checkout Mutation:
debugMessage: "Non-static method WPGraphQL\WooCommerce\Data\Mutation\Checkout_Mutation::update_session() cannot be called statically"

PHP Version 8

Does this close any currently open issues?

Fixes #472

Any relevant logs, error output, GraphiQL screenshots, etc?

(If it’s long, please paste to https://ghostbin.com/ and insert the link here.)

Any other comments?

…

Where has this been tested?

• WooGraphQL Version: …
• WPGraphQL Version: …
• WordPress Version:
• WooCommerce Version:

[kidunot89]

@manuelsampl Thanks for this. I'll include in the upcoming release.