fix: increase the default concurrency of Celery worker

[bowenliang123]

Checklist:

Important

Please review the checklist below before submitting your pull request.

• Please open an issue before creating a PR or link to an existing issue
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

Description

• before this PR, the async tasks often get blocked when only one worker instance started, as the default concurrency controlled by entrypoint.sh is set to 1
• considering the suggested default concurrency of celery worker command is the number of CPUs available on your system, it's reasonable to increasing to 3

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update, included: Dify Document
• Improvement, including but not limited to code refactoring, performance optimization, and UI/UX improvement
• Dependency upgrade

Testing Instructions

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Test A
• Test B

[bowenliang123]

cc @JohnJyong

[crazywoola]

See this one #6249

[bowenliang123]

See this one #6249

Cool~