docs: add scheduler migration known issue

[walteratsf]

Summary

Documents a known Scheduler concurrency behavior change when migrating applications from CloudHub to CloudHub 2.0.

• Explains that CloudHub 2.0 honors disallowConcurrentExecution, while affected CloudHub applications previously prevent overlapping executions even when the setting is false or omitted.
• Identifies affected schedulers: those with short intervals whose execution can exceed the configured interval.
• Describes the potential impact of overlapping executions on the shared application thread pool.
• Recommends explicitly setting disallowConcurrentExecution="true" to prevent overlaps after migration.
• Documents how to reduce repetitive Execution skipped log messages for short-interval schedulers.
• Adds links to the known issue from application migration considerations and the application-code migration checklist.

Testing

• Documentation-only change; reviewed rendered AsciiDoc structure and cross-references.

Writer's Quality Checklist

Before merging your PR, did you:

• Run spell checker
• Run link checker to check for broken xrefs
• Check for orphan files
• Perform a local build and do a final visual check of your content, including checking for:
  • Broken images
  • Dead links
  • Correct rendering of partials if they are used in your content
  • Formatting issues, such as:
    • Misnumbered ordered lists (steps) or incorrectly nested unordered lists
    • Messed up tables
    • Proper indentation
    • Correct header levels
• Receive final review and signoff from:
  • Technical SME
  • Product Manager
  • Editor or peer reviewer
  • Reporter, if this content is in response to a reported issue (internal or external feedback)
• If applicable, verify that the software actually got released