Rename 'spammable' to 'target'

What does this MR do?

Part of a general tidying of spam logic.

Spammable describes a type of object that can possibly be Spam.

AkismetMethods uses spammable as a name for the target object, but SpamLogs are not spammables - they are auxilliary objects. This MR renames spammable to target, but only in the context of direct usage with Akismet.

This is a retry of !24827 (merged), with a reduced scope. An end-to-end test has been added to avoid the regression it caused.

Screenshots

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

Related to #206895 (closed)