Update maintainer's handbook to reflect current contribution policies

[Copilot]

The maintainer's handbook contained outdated information about accepted repositories, dashboards, and meeting schedules that no longer exist.

Changes

• Repositories and projects: Limited accepted contributions to magento2 and magento2-page-builder only, removing references to pwa-studio, magento2-phpstorm-plugin, inventory, adobe-stock-integration
• Dashboard references: Removed mentions of non-existent "Community Dashboard" and "High Priority pull request Dashboard"
• Regular meetings: Replaced outdated meeting schedule with clarification that no regularly scheduled meetings currently exist

Example

Before:

Our maintainers work on numerous repositories within the Magento Github organization...
- [pwa-studio](https://developer.adobe.com/commerce/pwa-studio/)
- [magento2-phpstorm-plugin](https://github.com/magento/magento2-phpstorm-plugin)
- and [many others](https://github.com/magento)

After:

Currently, contributions are accepted from the following repositories:
- [magento2](https://github.com/magento/magento2)
- [magento2-page-builder](https://github.com/magento/magento2-page-builder)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Issue in /src/pages/guides/maintainers/handbook.md</issue_title>
<issue_description>The following improvements should be done:

Repositories and projects

• According to https://community.magento.com/t5/Magento-DevBlog/Guide-to-PR-Upvote/ba-p/563570, unfortunately, the contributions are currently accepted only from 2 repositories - https://github.com/magento/magento2 and https://github.com/magento/magento2-page-builder
• there is no such thing as a "High Priority pull request Dashboard" anymore

Pull request deployment tool

This deployment tool isn't exist anymore

Regular meetings

Maintainer’s Call – Once a month.
Community Contributions Triage – Scheduled twice a week.

these meeting doesn't exist for a long time</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Issue in /src/pages/guides/maintainers/handbook.md #183

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[dshevtsov]

@glo24157 please review

[github-actions]

⚠️ Warnings found but no fatal errors.

See the job summary for details.