Update guidance on new bot scopes.

### Your checklist for this issue

- [ ] Jenkins version

- [ ] Plugin version

- [ ] OS

### Description

Slack have added new scopes that might interest those who use a custom bot user.
Update guidance on the new bot scopes:

```
chat:write
Send messages as Jenkins

chat:write.public
Send messages to channels Jenkins isn't a member of

chat:write.customize
Send messages as Jenkins with a customized username and avatar

im:write
Start direct messages with people

reactions:write
Add and edit emoji reactions

users:read
View people in the workspace

users:read.email
View email addresses of people in the workspace
```

The once listed above might be the recommended scopes.

The issue names the Slack bot scopes to document but does not identify a documentation file or test. Locate the existing custom bot-scope guidance in the repository, update it with the listed scopes, and verify that the recommended scopes are clearly identified.

Written by the indexing model from the issue text.