Ensure that we are validating payload size for generic alerts

What does this MR do?

We were already checking this payload size before implicitly via NotificationPayloadParser which uses SafeInlineHash which uses Utils::DeepSize but this commit makes sure we are always (unconditionally) checking it.

Closes #14930 (closed). We can auto-close this issue since we are already safe on production and there's no way to verify this very MR 😅

Does this MR meet the acceptance criteria?

Conformity

• [-] 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team