Enable puma by default in GDK

What does this MR do?

This MR address gitlab-development-kit#490 (closed)

Enable puma by default in GDK. Change opt-in flag EXPERIMENTAL_PUMA to flag USE_WEB_SERVER. If USE_WEB_SERVER is undefined, use puma by default.

The implementation details: Move bin/web to bin/web_unicorn(removed PUMA switching logic) Introduce new shadow script bin/web. Now we have 3 scripts working together: bin/web, bin/web_puma, bin/web_unicorn

In bin/web, it check USE_WEB_SERVER:
   - if value is 'puma' or not set, call bin/web_puma
   - if value is 'unicorn', call bin/web_unicorn
   - report error if other values given

How to use unicorn after this MR: set environment variable USE_WEB_SERVER=unicorn before gdk run. For example, in the GDK root: echo "USE_WEB_SERVER=unicorn" >> .env or, USE_WEB_SERVER=unicorn gdk run

Due to documentation update for this change, this MR should be merged together with gitlab-development-kit!712 (merged)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes gitlab-development-kit#490 (closed)