fix(kong): don't populate KONG_ADMIN_GUI_SESSION_CONF only on OIDC

[czeslavo]

What this PR does / why we need it:

Fixes the problem discovered in KIC integration tests: https://github.com/Kong/kubernetes-ingress-controller/pull/6198.

KONG_ADMIN_GUI_SESSION_CONF should be not populated only if KONG_ADMIN_GUI_AUTH equals openid-connect. Restores the default value of enterprise.rbac.session_conf_secret to kong-session-config to avoid breaking changes (if someone relied on the default secret name as we did in KTF, https://github.com/Kong/charts/pull/1033 was a breaking change for them).

Checklist

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

[rainest]

@czeslavo this was marked as draft but it looks ready to go? Approved unmerged in case there was something remaining, but I figure it was just a misclick re status.