feat: Perform changes related to CI/CD. Prepare plugin for community migration.

[toninis]

Summary

• Remove scheduled ci workflow and upgrade plugin-ci reusable workflow
• Do not execute ci tasks upon tag creation
• Remove cd trigger upon workflow run event. Keep it simple upon tag creation.

Related PRs

https://github.com/mattermost/actions/pull/22 https://github.com/mattermost/actions-workflows/pull/40

Ticket Link

https://mattermost.atlassian.net/browse/CLD-7458

[hanzei]

@toninis Can you please merge the PR? I opened https://github.com/mattermost/mattermost-plugin-agenda/issues/113 to fix the broken CI linter.