fix: specifically set issuer ref group name

[kamontat]

What this PR does / why we need it:

This is the explicit group for cert-manager Issuer/ClusterIssuer.
On my use case is we set default issuer group to something different than default cert-manager.io

Special notes for your reviewer:

Checklist

• PR is based off the current tip of the main branch.
• Changes are documented under the "Unreleased" header in CHANGELOG.md
• New or modified sections of values.yaml are documented in the README.md
• Commits follow the Kong commit message guidelines

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[pmalek]

This change looks good 👍

Do you mind adding a changelog entry for this?

[kamontat]

@pmalek Added.