Remove SelfSigned issuer from released `transport-encryption` artifacts

[pierDipi]

Using a SelfSigned issuer is not recommended by cert-manager and currently we're shipping it in the tranport-encryption feature artifact.

Moving forward, we will not ship a SelfSigned issuer in the eventing-tls-networking.yaml artifacts and we will document, as a prerequisite, how to create a ClusterIssuer for eventing components so that by users can bring their own choosen issuer [1].

[1] https://cert-manager.io/docs/concepts/issuer/

Fixes #

Proposed Changes

• Remove SelfSigned issuer from released transport-encryption artifacts
• Add SelfSigned to the test configuration

Pre-review Checklist

• [ ] At least 80% unit test coverage
• [ ] E2E tests for any new behavior
• [ ] Docs PR for any user-facing impact
• [ ] Spec PR for any new API feature
• [ ] Conformance test for any change to the spec

Release Note

Remove `SelfSigned` issuer from released `transport-encryption` artifacts.
To continue using the `transport-encryption` feature, Eventing components require a pre-created `ClusterIssuer` with name `knative-eventing-ca-issuer`. For more details, refer to the `transport-encryption` feature documentation.

Docs

[pierDipi]

/cc @creydr @ReToCode

[creydr]

@pierDipi can you fix the boilerplate issue?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 69.13%. Comparing base (7e1c082) to head (0cbefa3). Report is 36 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7905      +/-   ##
==========================================
- Coverage   69.22%   69.13%   -0.09%     
==========================================
  Files         339      341       +2     
  Lines       19494    15829    -3665     
==========================================
- Hits        13494    10943    -2551     
+ Misses       5337     4213    -1124     
- Partials      663      673      +10     

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[pierDipi]

@creydr I've just copied this file from another placeholder, so I think it's ok to ignore the error there

[knative-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: creydr, pierDipi

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