build: add syslog as separate gem

[StephenHulme]

Follows on from #4647 with syslog.

Removes warning:

/Users/[user]/.rbenv/versions/3.3.6/lib/ruby/3.3.0/syslog/logger.rb:2: warning: /Users/[user]/.rbenv/versions/3.3.6/lib/ruby/3.3.0/arm64-darwin24/syslog.bundle was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.4.0.
You can add syslog to your Gemfile or gemspec to silence this warning.

Changes proposed in this pull request

• Add syslog as separate dependency in Gemfile

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.34%. Comparing base (29a2e2c) to head (b80add8).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4662      +/-   ##
===========================================
- Coverage    89.43%   89.34%   -0.09%     
===========================================
  Files         1406     1406              
  Lines        30296    30296              
===========================================
- Hits         27095    27069      -26     
- Misses        3201     3227      +26     

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