core.setOutput function will be disabled 1st June 2023 unless actions/core is updated.

[bmos]

The set-output command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

Now obviously you're not using stdout, but you are using the core.setOutput function: https://github.com/frabert/replace-string-action/blob/9b62dfe3ae936b1cc380f2421c8ac9e63a4a3e85/index.js#L11

Which according to the linked blog post, means you need to update actions/core to at least v1.10.0:

Action authors who are using the toolkit should update the @actions/core package to v1.10.0 or greater to get the updated saveState and setOutput functions.

It seems currently to be 1.4.0 as of this prior issue which discussed how to update it!