chore(ci): don't test istio against k8s v1.26

[czeslavo]

What this PR does / why we need it:

According to Istio's support status, Kubernetes v1.26 isn't supported by any Istio version.

Which issue this PR fixes:

Part of #3199

[randmonkey]

https://istio.io/latest/docs/releases/supported-releases/
I think k8s 1.25 is not officially supported by istio 1.14 and earlier.

[czeslavo]

https://istio.io/latest/docs/releases/supported-releases/ I think k8s 1.25 is not officially supported by istio 1.14 and earlier.

Yeah, but I think as long as our tests pass in this configuration, we're ok, aren't we?

[codecov]

Codecov Report

Base: 74.1% // Head: 74.0% // Decreases project coverage by -0.0% ⚠️

Coverage data is based on head (0928973) compared to base (48f4d7a).
Patch has no changes to coverable lines.

❗ Current head 0928973 differs from pull request most recent head 886310d. Consider uploading reports for the commit 886310d to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3280     +/-   ##
=======================================
- Coverage   74.1%   74.0%   -0.1%     
=======================================
  Files        111     111             
  Lines      13275   13275             
=======================================
- Hits        9847    9834     -13     
- Misses      2801    2810      +9     
- Partials     627     631      +4     

Impacted Files	Coverage Δ
...ternal/controllers/gateway/httproute_controller.go	52.5% <0.0%> (-1.7%)	⬇️
internal/dataplane/parser/parser.go	89.7% <0.0%> (-1.7%)	⬇️
internal/dataplane/kongstate/service.go	67.3% <0.0%> (+1.2%)	⬆️

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