Document for 3.6 release

[ahrtr]

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We need to cover the following topics,

• Release blogs

  • Some features: v2store deprecation, downgrade, etc.
  • performance improvement (reduce memory usage):
    • https://github.com/etcd-io/etcd/pull/18825
    • https://github.com/etcd-io/etcd/pull/17563

• Upgrade checklist and guide

  • v2store deprecation: --enable-v2, --experimental-enable-v2v3 and other v2 proxy flags have already been removed in 3.6. Refer to https://github.com/etcd-io/etcd/pull/18999
    • Users must remove these these flags before upgrading from 3.5 to 3.6. Also refer to https://github.com/etcd-io/etcd/issues/17009#issuecomment-1838965996
    • Run the tool to ensure there is no custom content in v2store: https://github.com/etcd-io/etcd/issues/18993
  • downgrade stuff
  • anything else

• The breaking change on the experimental flag as mentioned in https://github.com/etcd-io/etcd/issues/18023#issuecomment-2653717397

• anything else ?

cc @jmhbnz @ivanvc