fix: try recovering from gateway sync failure only on UpdateError

[czeslavo]

What this PR does / why we need it:

We should try recovering with the fallback configuration (either generated or the persisted last valid one) only if we get an expected UpdateError from the UpdateStrategy. Otherwise, we can assume the error is unrecoverable through the fallback configuration (e.g. transient network errors, gateways returning 5xx, etc.).

It relies on Kong/go-kong#452.

Which issue this PR fixes:

Fixes #6218.

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.

Project coverage is 39.1%. Comparing base (3fa3bc7) to head (15f7a40).

❗ Current head 15f7a40 differs from pull request most recent head 16ce72a

Please upload reports for the commit 16ce72a to get more accurate results.

Files	Patch %	Lines
internal/dataplane/kong_client.go	0.0%	14 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #6237      +/-   ##
========================================
- Coverage   73.6%   39.1%   -34.5%     
========================================
  Files        200     199       -1     
  Lines      19909   19903       -6     
========================================
- Hits       14654    7798    -6856     
- Misses      4264   11168    +6904     
+ Partials     991     937      -54     

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