Backport of Improve ux around ACL token to help users avoid overwriting node/service identities into release/1.15.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #16506 to be assessed for backporting due to the inclusion of the label backport/1.15.

The below text is copied from the body of the original PR.

---

Description

Follow up to #16288
TLDR: node-identity and service-identity flags when specified without the merge-* flags overwrites service identities and node identities of a token. This is ambiguous to the user because we have a no-merge flag which seems to imply that on update we are merging. So we have been doing work to improve the ux by renaming/adding/sunsetting flags and adding more docs to help reduce the ambiguity

• Marking merge-node-identities and merge-service-identities for deprecation and encouraging the use of their replacement append-node-policy and append-service-identity
• Updating docs to clarify that using -rules overwrites existing rules
• Adding docs for the new flags: append-node-policy and append-service-identity

PR Checklist

• updated test coverage
• external facing docs updated
• not a security concern

---

Overview of commits

• 04a7185 - f47fbf7 - bf9fb37 - 22fde76 - 0313fa6 - 6e19413 - e1fb12f - 4beecd1

[github-team-consul-core-pr-approver]

Auto approved Consul Bot automated PR