feat(gateway_discovery): allow gateway discovery only in dbless mode

[randmonkey]

What this PR does / why we need it:

Allow gateway discovery only in db-less mode

Which issue this PR fixes:

fixes #3401
Special notes for your reviewer:

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

Patch coverage: 75.0% and project coverage change: +0.1 🎉

Comparison is base (afb93b3) 72.9% compared to head (ab3415c) 73.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3642     +/-   ##
=======================================
+ Coverage   72.9%   73.0%   +0.1%     
=======================================
  Files        131     131             
  Lines      15723   15735     +12     
=======================================
+ Hits       11469   11495     +26     
+ Misses      3494    3483     -11     
+ Partials     760     757      -3     

Impacted Files	Coverage Δ
internal/manager/run.go	36.0% <25.0%> (-0.2%)	⬇️
internal/manager/config_validation.go	89.4% <100.0%> (+1.1%)	⬆️
internal/dataplane/parser/parser.go	89.1% <0.0%> (+0.6%)	⬆️
...nternal/controllers/gateway/tlsroute_controller.go	73.4% <0.0%> (+2.0%)	⬆️
...nternal/controllers/gateway/udproute_controller.go	73.7% <0.0%> (+2.1%)	⬆️

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