Improve dex kustomizations readability

[kromanow94]

The overlays for ldap and github were not taken into consideration because atm they don't work because the dex's base kustimization doesn't define dex-parameters CM anymore.

These changes shouldn't cause any differences in generated manifests.

Description of your changes:
Implementing changes discussed here: #2586 (comment)

Checklist:

• test if there are no differences in generated manifest

  $ git reset --hard
  HEAD is now at 2d047ae6 Improve dex kustomizations readability
  
  $ kustomize build common/dex/overlays/istio/ > dex.overlay.istio.improve-dex-kustomization-readability.yaml
  $ kustomize build common/dex/base/ > dex.overlay.base.improve-dex-kustomization-readability.yaml
  
  $ git checkout master
  $ git log -1
  commit ed4caf3e26060f626571b421185326257777576c (HEAD -> master, origin/master)
  Author:     Aleksey Karpov <[email protected]>
  AuthorDate: Tue Jan 2 19:49:15 2024 +0300
  Commit:     GitHub <[email protected]>
  CommitDate: Tue Jan 2 16:49:15 2024 +0000
  
      Fixes dex for kustomize 5 (#2586)
  
      * Fixes user-namespace for kustomize 5
  
      * Fixes dex for kustomize 5
  $ kustomize build common/dex/base/ > dex.overlay.base.master.yaml
  $ kustomize build common/dex/overlays/istio/ > dex.overlay.istio.master.yaml
  
  $ diff dex.overlay.base.improve-dex-kustomization-readability.yaml dex.overlay.base.master.yaml
  $ diff dex.overlay.istio.improve-dex-kustomization-readability.yaml dex.overlay.istio.master.yaml
  

[juliusvonkohout]

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: juliusvonkohout, kromanow94

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