fix: fix a panic when receiving a broken config from Gateway

[pmalek]

What this PR does / why we need it:

When KIC receives a broken config from Gateway (e.g. like the one described in #4990) with route's priority in scientific notation:

routes:
  - request_buffering: true
    expression: ((http.path == "/echo") || (http.path ^= "/echo/")) && (http.method == "GET")
    preserve_host: true
    name: default.echo.echo..1027
    response_buffering: true
    updated_at: 1698340425
    https_redirect_status_code: 426
    ws_id: 0dc6f45b-8f8d-40d2-a504-473544ee190b
    id: 6ece5df4-2c97-5372-a840-47e222d4ea9d
    created_at: 1698340425
    tags:
      - k8s-name:echo
      - k8s-namespace:default
      - k8s-kind:Ingress
      - k8s-uid:6eb54997-0c70-4a68-918b-935a356f14cd
      - k8s-group:networking.k8s.io
      - k8s-version:v1
    priority: 3.3820977671045e+15 # <- this
    service: 4a3125fc-7d51-5575-b0f3-978d9a44a49b
    protocols:
      - http
      - https
    strip_path: true

then it cannot handle an unparsed configuration and it panics:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x18 pc=0x1037d8a34]

goroutine 138 [running]:
github.com/kong/kubernetes-ingress-controller/v2/internal/dataplane/configfetcher.KongRawStateToKongState(0x0)
        kubernetes-ingress-controller/internal/dataplane/configfetcher/kongrawstate.go:19 +0x84
github.com/kong/kubernetes-ingress-controller/v2/internal/dataplane/configfetcher.(*DefaultKongLastGoodConfigFetcher).TryFetchingValidConfigFromGateways(0x14000aad1d0, {0x1040464d0, 0x14000aacff0}, {{0x104049b88?, 0x140000db230?}, 0x0?}, {0x14000bd3100, 0x2, 0x0?})
        kubernetes-ingress-controller/internal/dataplane/configfetcher/config_fetcher.go:85 +0x438
github.com/kong/kubernetes-ingress-controller/v2/internal/dataplane.(*KongClient).Update(0x140009ec1e0, {0x1040464d0, 0x14000aacff0})
        kubernetes-ingress-controller/internal/dataplane/kong_client.go:397 +0x124
github.com/kong/kubernetes-ingress-controller/v2/internal/dataplane.(*Synchronizer).startUpdateServer(0x14000579340, {0x1040464d0, 0x14000aacff0})
        kubernetes-ingress-controller/internal/dataplane/synchronizer.go:183 +0xa0
created by github.com/kong/kubernetes-ingress-controller/v2/internal/dataplane.(*Synchronizer).Start in goroutine 136
        kubernetes-ingress-controller/internal/dataplane/synchronizer.go:120 +0x1c8

This PR allows KIC to handle this error and print a log about it:

2023-10-27T19:52:11+02:00     error   failed to fetch last good configuration from gateways   {"error": "routes: json: cannot unmarshal number 3.379898743849e+15 into Go struct field Route.priority of type int\nfailed to fetch configuration from \"https://10.244.0.241:8444\", got nil kong raw state\nroutes: json: cannot unmarshal number 3.379898743849e+15 into Go struct field Route.priority of type int\nfailed to fetch configuration from \"https://10.244.0.242:8444\", got nil kong raw state"}

Special notes for your reviewer:

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[programmer04]

🎖️

[czeslavo]

👍

[team-k8s-bot]

The backport to release/2.12.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release/2.12.x release/2.12.x
# Navigate to the new working tree
cd .worktrees/backport-release/2.12.x
# Create a new branch
git switch --create backport-5003-to-release/2.12.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 477e93229d3d29ac0ef918333468b3e1ed0b5b81
# Push it to GitHub
git push --set-upstream origin backport-5003-to-release/2.12.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release/2.12.x

Then, create a pull request where the base branch is release/2.12.x and the compare/head branch is backport-5003-to-release/2.12.x.