Replace `set-output` usage with `GITHUB_OUTPUT`

First of all, thank you for this action! It's been very useful.

I noticed while using it that it's using `set-output` internally, which has been deprecated by GitHub:

> 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/

GitHub's encouraged alternative is to use the `$GITHUB_OUTPUT` file, e.g.:

```bash
echo "foo=bar" >> "${GITHUB_OUTPUT}"
```

I'm not super familiar with Go, but I suspect you're using the deprecated `set-output` command indirectly through this line:

https://github.com/guilhem/rss-issues-action/blob/74cacbcc0b4d2cf0879b3141d6e0f87c32ca270a/main.go#L242

...and that upgrading to `1.1.0` or newer will fix the underlying implementation, per https://github.com/sethvargo/go-githubactions/pull/49