feat: make expressions router default

[czeslavo]

What this PR does / why we need it:

Sets the expression router as default in kong/kong. Adds validations that ensure that:

• expressions router flavor is not used with KIC < 2.10 as it's not supported
• when expressions router flavor is used with KIC < 3.0, the feature gate must be set.

to both kong/kong and kong/ingress charts.

Validations will output instructions on how to fix the issue if it is encountered.

It also changes the way we refer to kong/kong in kong/ingress's Chart.yaml: instead of using the repository URL that forces us to bump charts in two steps, use file:// to make it possible in one step.

Which issue this PR fixes

Should solve https://github.com/Kong/kubernetes-ingress-controller/issues/5129.

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• [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

[czeslavo]

Per @mheap, I'm labeling this with HOLD/DO NOT MERGE until Konnect is compatible with expressions.

[pmalek]

Could potentially be put in kong v3 milestone