chore: serverMiddleware HMR improvements

[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

Followed by #6881:

• Improve route fallback strategy. Always preserve previous route if no new one provided
• Fix support for express apps as middleware. (connect behaviour is undocumented so i used a simpler approach)

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 #6896 into dev will decrease coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##              dev    #6896      +/-   ##
==========================================
- Coverage   62.62%   62.61%   -0.02%     
==========================================
  Files          82       82              
  Lines        3291     3295       +4     
  Branches      894      896       +2     
==========================================
+ Hits         2061     2063       +2     
- Misses        988      989       +1     
- Partials      242      243       +1

Flag	Coverage Δ
#e2e	?
#unit	?
#unittests	62.61% <66.66%> (+0.01%)	⬆️

Impacted Files	Coverage Δ
packages/server/src/server.js	83.89% <66.66%> (-0.25%)	⬇️
packages/utils/src/route.js	96.55% <0%> (-0.69%)	⬇️

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