Backport #4097 to rc/v0.111.x

[eval-exec]

What problem does this PR solve?

Related issue: #4092

Backport #4097

Problem Summary:

What's Changed:

Related changes

• let epoch_duration_target affect epoch length when permanent_difficulty_in_dummy is enabled in Dummy mode.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code ci-runs-only: [ quick_checks,linters ]

Side effects

• Performance regression
• Breaking backward compatibility

Release note

Note: This PR introduce a breaking change for `Dummy` mode. 
When the CKB is in `Dummy` mode and the `permanent_difficulty_in_dummy` option is set to `true`,
the epoch length will be influenced by the `epoch_duration_target` parameter.
Specifically, the epoch length will be calculated as `epoch_duration_target / 8`. 

For example, if `epoch_duration_target` is configured as `14400`,
the resulting epoch length will be calculated as: `14400` / `8` = `1800`.