fix: restore defaulting to protocol "grpcs" for gRPC Service

[programmer04]

What this PR does / why we need it:

Heuristic introduced in #5128 for guessing whether for Kong Service grpc or grpcs protocol should be set breaks default behavior, that service without annotation for GRPCRoute is treated as grpcs.

Now documentation always mentions annotations for both grpc and grpcs see

• kic: update and extend gRPC how-to docs.konghq.com#6567
  but before when only gRPC over HTTPS was supported annotations weren't mentioned. Thus users probably haven't configured it and we have to avoid breaking their setups. This PR ensures that such a configuration will work.

Which issue this PR fixes:

Part of #4273

Special notes for your reviewer:

• e0d56ea - modifies the test to show that the assumption about grpcs is indeed broken (the same setup as described in the docs before the update)
• 6b2764b - contains actual fix (reverting heuristic introduced in fix: support GRPC with Gateway's HTTP listener #5128 - see getProtocolForKongService(...) and some small clean-ups and improvements

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

All modified and coverable lines are covered by tests ✅

Comparison is base (96941c3) 54.6% compared to head (db9f680) 75.5%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #5283      +/-   ##
========================================
+ Coverage   54.6%   75.5%   +20.9%     
========================================
  Files        169     170       +1     
  Lines      19264   19370     +106     
========================================
+ Hits       10530   14638    +4108     
+ Misses      7794    3903    -3891     
+ Partials     940     829     -111     

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