[Backport 2.x] [Bug] Make release note generation more resilient by gracefully handling invalid changelog fragments

[opensearch-trigger-bot[bot]]

Backport ab4e6e8a0d45030a13c170bf8337fc06772c4ada from #8780.

[github-actions[bot]]

❌ Invalid Changelog Heading

The '## Changelog' heading in your PR description is either missing or malformed. Please make sure that your PR description includes a '## Changelog' heading with proper spelling, capitalization, spacing, and Markdown syntax.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 60.98%. Comparing base (07063ff) to head (d846bf6). Report is 5 commits behind head on 2.x.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 2.x #8881 +/- ## ======================================= Coverage 60.98% 60.98% ======================================= Files 3799 3799 Lines 90856 90856 Branches 14306 14306 ======================================= + Hits 55411 55413 +2 + Misses 31930 31929 -1 + Partials 3515 3514 -1 ``` | [Flag](https://app.codecov.io/gh/opensearch-project/OpenSearch-Dashboards/pull/8881/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | Coverage Δ | | |---|---|---| | [Linux_1](https://app.codecov.io/gh/opensearch-project/OpenSearch-Dashboards/pull/8881/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | `29.03% <ø> (ø)` | | | [Linux_2](https://app.codecov.io/gh/opensearch-project/OpenSearch-Dashboards/pull/8881/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | `56.53% <ø> (ø)` | | | [Linux_3](https://app.codecov.io/gh/opensearch-project/OpenSearch-Dashboards/pull/8881/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | `37.88% <ø> (+<0.01%)` | :arrow_up: | | [Linux_4](https://app.codecov.io/gh/opensearch-project/OpenSearch-Dashboards/pull/8881/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | `30.00% <ø> (ø)` | | | [Windows_1](https://app.codecov.io/gh/opensearch-project/OpenSearch-Dashboards/pull/8881/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | `29.05% <ø> (ø)` | | | [Windows_2](https://app.codecov.io/gh/opensearch-project/OpenSearch-Dashboards/pull/8881/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | `56.48% <ø> (ø)` | | | [Windows_3](https://app.codecov.io/gh/opensearch-project/OpenSearch-Dashboards/pull/8881/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | `37.88% <ø> (ø)` | | | [Windows_4](https://app.codecov.io/gh/opensearch-project/OpenSearch-Dashboards/pull/8881/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project) | `30.00% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opensearch-project#carryforward-flags-in-the-pull-request-comment) to find out more.

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