Add section `v2store is finally gone` into etcd v3.8 release blog

## Title

`V2store is finally gone`

## Body

Eleven years in the making. etcd introduced v3store back in v2.1 (May 2015) as a from-scratch redesign — a flat, MVCC key-value model built for correctness and performance, replacing the json-structured v2store. But ripping out v2store itself couldn't happen overnight: it meant carrying two storage engines side by side for years, phasing out the v2 API, deprecating v2 snapshots, teaching etcd to bootstrap entirely from v3store instead of from v2 snapshot state ([#20187](https://github.com/etcd-io/etcd/issues/20187)), and finally cleanup everything related to v2 snapshot. Each step spanned multiple major and minor releases, done carefully so existing clusters could upgrade without disruption.

As of today, that decade-plus effort is done. etcd no longer generates or loads v2 snapshot files, all the legacy v2 snapshot code is gone, and any leftover v2 snapshot files are cleaned up automatically on bootstrap.

Before you upgrade: remove the --max-snapshots and --v2-deprecation flags — they no longer exist in v3.8, and etcd won't start with them set.

Planning to downgrade? v3.8 → v3.7 works normally. If you need to go further, to v3.6, wait until v3.7 has generated at least one v2 snapshot before continuing the downgrade.

Start by locating the etcd v3.8 release blog in the website repository and review its existing section structure. Add the supplied “V2store is finally gone” section in the appropriate place, then preview or build the site to confirm the formatting, links, and upgrade notes render correctly.

Written by the indexing model from the issue text.