Add 10.2.0 changelog and release notes (#13521)

* Add 10.2.0 changelog and release notes

Generate CHANGELOG-10.2.0 from the 10.2.0 milestone and document the
release in whats-new and upgrading. The connect retry change (#13102)
is called out as a necessary incompatible change, since the retry
limits were not previously applied according to origin state.

* Address review: fix PR count and token_key markup

The PR count was 655 before five stale milestone entries were dropped;
the changelog has 650. Use :ts:cv: for
proxy.config.quic.server.token_key.filename, which is documented on
10.2.x even though it is absent from master, where it was first checked.

* Add late 10.2.x additions to changelog and release notes

Picks up #13328 (shared-memory cache directory for fast restart) and
#13418 (traffic_ctl cache clear). The shm directory gets its own section
since it is a new opt-in feature with four new records and a traffic_ctl
subcommand.

* Add July 2026 security fixes to changelog and release notes

The Release 2 security bundle (#13452) landed directly on 10.2.x without
public PRs, so those commits never appear in a milestone. Source them
from the commit range with the changelog tool's git-range mode and append
them as bare subjects, matching how CHANGELOG-10.1.4 lists them. Link
the advisory from whats-new for the CVE mapping.

* Add #13352, #13517 and #13523 to the changelog

Three late bug fixes on 10.2.x. All are fixes with no new configuration,
metrics or API surface, so only the changelog and the commit/PR counts
change.

Author: cmcfarlen