releases: correct release dates published one day early

[bosconi]

Seven sections in the release notes give release dates one day early. Release notes use the day of the US rollout (Thursday) as the Cloud date and the day the Helm chart is published (Friday) as the Self-Managed date. From v26.35.0 on, the generated notes used the final tag's date instead, and that tag is cut on Wednesday. The generator is fixed going forward by MaterializeInc/mz-skills#301.

Section	Cloud	Self-Managed	Evidence
v26.44.1	09-30 → 10-01	10-01 → 10-02	US rollout 10-01; Helm 10-02
v26.43.0	09-23 → 09-24	09-24 → 09-25	US rollout 09-24; Helm 09-25
v26.40.0	09-02 → 09-03	09-03 → 09-04	US rollout 09-03; Helm 09-04
v26.39.0	08-26 → 08-27	08-27 → 08-28	US rollout 08-27; Helm 08-28
v26.38.2	08-19 → 08-20	08-25 (unchanged)	US rollout of v26.38.0 08-20
v26.37.0	08-12 → 08-13	08-13 → 08-14	US rollout 08-13; Helm 08-14
v26.35.0	07-29 → 07-30	07-30 → 07-31	US rollout 07-30; Helm 07-31

The operator compatibility rows for v26.44.1, v26.43, v26.40, v26.39, v26.37 and v26.35 move to the same Self-Managed dates. The US rollout dates come from the upgrade-production-us runs in #release. The Helm dates come from the chart's created timestamps in index.yaml on gh-pages.

Left alone; may be worth a look:

• v26.40.2 says Cloud 09-07 / Self-Managed 09-08, but its Helm chart was published 09-10.
• v26.41.0's Helm chart is stamped 09-13 04:08 UTC, which is a day after the Self-Managed date of 09-11.
• The v26.38 compatibility row (08-20) has no published v26.38.0 chart.

Merge order: this PR and #39661 (v26.45.1) both edit the lines right below the top of _index.md and the compatibility YAML. Whichever merges second will need a trivial rebase.

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