Fix issue hook events text

What does this MR do?

The webhook admin page previously said that issue webhooks could fire when issues were merged. Issues can't be merged, however, our docs say that the hooks do fire on issue create, update, close or reopen. This MR updates the copy to reflect the docs.

QA

Visit Project > Settings > Webhooks to compare the copy changes to Issues events and Confidential issues events. Note the shared partials also display for Group webhooks.

Screenshots (strongly suggested)

Before	After

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done