Install libpcre2-dev via package manager

What does this MR do?

Our installation documentation instructs users to install libpcre2 by manually compiling it. This has been required in the past as it wasn't available in some conservative distributions like Debian, but nowadays it is available. Most importantly, Ubuntu 16.04 and Debian 9 both have it in their repositories, which are the oldest versions which are still supported.

Drop these instructions in favor of installing libpcre2-dev via the package manager.

Related to gitaly#2729 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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