Enable feature flag "errors_utf_8_encoding" by default

What does this MR do and why?

Contributes to #422061 (closed)

Problem

Errors returned from Gitaly are encoded in ASCII. If the error message contains non-ASCII characters, then it returns an encoding error.

Solution

Encode Gitaly error messages in UTF-8.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.