docs: update release calendar for v2.38 - #30757

Updates the release calendar in docs/reference/releases.md.

For the 2.38 release:

• 2.38 becomes Mainline
• 2.37 Stable
• 2.36 Security Support
• 2.35 Not Supported
• 2.32 drops off.

Latest patch versions are bumped to v2.34.12, v2.35.9, v2.36.7, v2.37.4, and v2.38.0, and a 2.39 "Not Released" row is added.
The 2.35 row's link text (v2.35.6) is fixed to match its target (v2.35.7, now v2.35.9).

Generated with scripts/update-release-calendar.sh, then hand-adjusted:

• 2.38 links to its changelog post, broader-model-access-and-tighter-governance-controls-in-238, since coder-2-38 returns 404. The longer link widens the table, so every row is realigned.
• 2.35 keeps coder-2-35-1; the script's coder-2-35 returns 404.
• 2.38's date is the GitHub release publish date (October 06, the first Tuesday). The script uses the tag commit date (October 01).

The "latest ESR version" link in the ESR section is also updated from v2.34.5 to v2.34.12.

Note: re-running the script will revert the two link overrides above.

Generated by Coder Agents on behalf of @Lisa-Fiander.