test: use EG v1.6.0 rc1

[mathetake]

Description
Envoy Gateway v1.6.0-rc.1 has been released a few days ago, so this commit uses it for our testing to prepare for our next release: https://github.com/envoyproxy/gateway/releases/tag/v1.6.0-rc.1

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.99%. Comparing base (c87e367) to head (da96e0a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1471   +/-   ##
=======================================
  Coverage   83.99%   83.99%           
=======================================
  Files         142      142           
  Lines       12555    12555           
=======================================
+ Hits        10545    10546    +1     
+ Misses       1400     1398    -2     
- Partials      610      611    +1     

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