actions: Use variables rather than secrets for non-secret data

[anomiex]

Closes MONOREP-300

Proposed changes:

We don't need the project board URL or various Slack channel IDs to be secret. They're just identifiers, and not even ones where security-through-obscurity might be beneficial.

Pairs like ID/key are left as secrets, even if the ID isn't really secret, because it's clearer to keep those together than to split them across two different lists.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

p1764014167264399-slack-C05Q5HSS013

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Check that the variables are defined at https://github.com/Automattic/jetpack/settings/variables/actions, with appropriate values.
• We can check that the E2E reporting works from the PR. The rest would need a properly set up fork.

[github-actions]

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