docs: clarify Envoy and dataplane LTS support policy

[zalimeni]

Update matrices and clarify statements as to when Consul expands support to new major versions of Envoy and Consul dataplane in light of Consul LTS or Envoy EOL status.

PR Checklist

• [ ] updated test coverage
• [x] external facing docs updated
• [x] appropriate backport labels added
• [x] not a security concern

[zalimeni]

@blake @jkirschner-hashicorp bumping this for your review so it doesn't get lost in the stale PR pile. Not urgent but would love to get it merged if we can soon. Thanks!

[zalimeni]

Manually backporting to 1.19 and 1.18 LTS docs in CE after merge.