fix(vue-renderer): add vary header for user-agent in modern server mode

[pi0]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

When running in modern server mode, our response varies depending on user-agent. A reverse-proxy may wrongly cache the modern/legacy response and use it for others. This PR tries to prevent that.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.

[codecov-io]

Codecov Report

Merging #5807 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #5807      +/-   ##
==========================================
+ Coverage   95.67%   95.68%   +<.01%     
==========================================
  Files          82       82              
  Lines        2682     2686       +4     
  Branches      687      688       +1     
==========================================
+ Hits         2566     2570       +4     
  Misses         98       98              
  Partials       18       18

Flag	Coverage Δ
#e2e	100% <ø> (ø)	⬆️
#fixtures	50.33% <0%> (-0.08%)	⬇️
#unit	92.66% <100%> (+0.01%)	⬆️

Impacted Files	Coverage Δ
packages/vue-renderer/src/renderers/modern.js	98.33% <100%> (+0.11%)	⬆️

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