KEP-740: Update with decision on max token lifetime

[HarshalNeelkamal]

• One-line PR description:
  Choice of max token lifetime was reconsidered during implementation. Documenting it accordingly.

• Issue link:
  API for external signing of Service Account tokens #740

• Other comments:
  For ref: Add plugin and key-cache for ExternalJWTSigner integration kubernetes#128190 (comment)

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @HarshalNeelkamal. Thanks for your PR.

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[HarshalNeelkamal]

cc/ @liggitt