docs: add v3.6 to v3.7 upgrade guide

[nicknikolakakis]

Fixes #1165

Adds content/en/docs/v3.7/upgrades/upgrade_3_7.md and links it from upgrading-etcd.md.

What the guide covers

Per the requirements in #1165:

• 3.6.11+ prerequisite called out in an alert/warning block.
• Removed experimental flags: lists all 23 --experimental-* flags removed in v3.7 (#19959) and points readers at the v3.5 to v3.6 guide for the non-experimental / --feature-gates mapping (since each one was already deprecated in 3.6).
• Final v2store removal: notes that --enable-v2, --experimental-enable-v2v3, v2 discovery, client/v2, and v2 snapshot file loading are all gone in 3.7 (#20109, #20117, #21263) and that this is a no-op for anyone coming from a clean 3.6.
• Go refactoring: links #14533 (gogo/protobuf -> google.golang.org/protobuf), #20420 (grpc-middleware v1 -> v2), and #20017 (otelgrpc v0.61.0). Flagged as not affecting normal upgraders but worth checking for integrators.
• Downgrade link: rollback section points to downgrades/downgrading-etcd as the "in case your upgrade goes badly" path.

The Flags added / Flags with new defaults sections are explicitly "None" - confirmed by diffing etcd --help output between v3.6.12 and v3.7.0-rc.0.

Tested

The upgrade procedure example is not synthetic. I ran a real rolling upgrade of a 3-node etcd v3.6.12 cluster to v3.7.0-rc.0 on darwin/arm64 (released v3.7.0-rc.0 binaries from the GitHub release) and captured the actual log lines, endpoint status output, and version transitions. Specifically I verified:

1. Healthy 3-node v3.6.12 cluster started; test keys written; snapshot backup taken.
2. Mid-upgrade mixed state: one v3.7.0-rc.0 + two v3.6.12 members reported etcdcluster: 3.6.0 (lowest common protocol).
3. After all three were upgraded, the cluster version auto-advanced to 3.7.0 and storage version advanced to 3.7.0.
4. All keys written pre-upgrade preserved; new writes succeeded post-upgrade.

Local site build

npm install && npx hugo --cleanDestinationDir -e dev -DFE builds cleanly. The new page renders at /docs/v3.7/upgrades/upgrade_3_7/, appears in the sidebar, and all internal links resolve (upgrade_3_6, op-guide/maintenance, downgrades/downgrading-etcd, v3.4/op-guide/v2-migration).

markdownlint-cli2 produces the same single MD001 finding as the existing upgrade_3_6.md, so I kept the heading structure consistent with the rest of the upgrade-guide family.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @nicknikolakakis. Thanks for your PR.

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[jberkus]

Wow, nice! Reviewing.