Update openshift requirements under docs

[michael-valdron]

Description of Changes

Updates OpenShift requirements under the documentation to specify 4.15.x-4.18.x compatibility and indicate that the Operator is actively tested on OpenShift v4.18.

Related Issue(s)

resolves devfile/api#1669

Acceptance Criteria

Tests

• Test Coverage
  • Are your changes sufficiently tested, and are any applicable test cases added or updated to cover your changes?
• Gosec scans
  • Fix all MEDIUM and higher findings and/or annotate a finding per gosec instructions: https://github.com/securego/gosec#annotating-code to address why a finding is not a security issue

Documentation

• Does the registry operator documentation need to be updated with your changes?

Tests Performed

Ran make test-integration on an active deployment of the current state of main. Tested against OpenShift 4.16-4.18.

How To Test

Running Unit Tests

Running Integration Tests

Notes To Reviewer

[michael-valdron]

/hold

Waiting for v4.18 testing setup to be merged: openshift/release#66235

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 34.27%. Comparing base (7b69290) to head (cad92a8).
Report is 18 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #106      +/-   ##
==========================================
+ Coverage   28.34%   34.27%   +5.93%     
==========================================
  Files          25       30       +5     
  Lines        1450     1838     +388     
==========================================
+ Hits          411      630     +219     
- Misses       1018     1184     +166     
- Partials       21       24       +3     

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[thepetk]

\lgtm

[Jdubrick]

nit but doesn't need to be added: should we link to the kube api and openshift 4.18

[michael-valdron]

@Jdubrick I've added links to the release notes of these versions: 629ad36

[michael-valdron]

/unhold

openshift/release#66235 has been merged.

[Jdubrick]

lgtm

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Jdubrick, michael-valdron, thepetk

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