docs(e2e): sync README with release/1.3.x defaults

- OLD_VERSION examples: v12.0.1 (stale evmos-era tag, never valid on
  any moca release) -> v1.2.1, matching the script + workflow defaults
  on this branch (aef131b / a5634ae). Three occurrences in
  e2e/kind/README.md.
- CI section: workflow runs automatically on PRs to `main` -> PRs to
  `release/1.3.x`, matching the workflow's pull_request trigger filter
  (a5634ae).

Resolves phenix3443's follow-up review point 2 on #255.

Author: puneet2019

e2e/kind/README.md

@@ -36,7 +36,7 @@ make e2e-fw-test TEST=upgrade_hardfork
 make e2e-fw-test TEST=upgrade_governance
 
 # Run with OLD_VERSION for upgrade tests
-OLD_VERSION=v12.0.1 make e2e-fw-test TEST=upgrade_hardfork
+OLD_VERSION=v1.2.1 make e2e-fw-test TEST=upgrade_hardfork
 
 # Dev mode (skip cleanup, leave cluster running for debugging)
 make e2e-fw-dev TEST=smoke
@@ -109,15 +109,15 @@ Tests a binary upgrade from an old version to the current build using the app.to
 
 ```bash
 # Upgrade from a specific release (pulls/builds old version, deploys, upgrades, validates)
-OLD_VERSION=v12.0.1 make e2e-kind-upgrade-hardfork
+OLD_VERSION=v1.2.1 make e2e-kind-upgrade-hardfork
 ```
 
 ### Upgrade Tests (governance mode)
 
 Same as hardfork mode but triggers the upgrade via a governance software-upgrade proposal.
 
 ```bash
-OLD_VERSION=v12.0.1 make e2e-kind-upgrade-governance
+OLD_VERSION=v1.2.1 make e2e-kind-upgrade-governance
 ```
 
 ## Step by Step
@@ -302,7 +302,7 @@ curl http://localhost:26657/net_info | jq '.result.n_peers'
 
 ## CI
 
-The GitHub Actions workflow (`.github/workflows/e2e-kind.yml`) runs automatically on PRs to `main`. It:
+The GitHub Actions workflow (`.github/workflows/e2e-kind.yml`) runs automatically on PRs to `release/1.3.x`. It:
 
 1. Installs Kind
 2. Builds Docker image and deploys to Kind