fix(webhook-service): Fix retrieval of webhook config

[bacherfl]

Backport for #7094

Integration test run: https://github.com/keptn/keptn/runs/5508914080?check_suite_focus=true

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #7124 (5b03578) into 0.13.x (5053d5c) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           0.13.x    #7124      +/-   ##
==========================================
+ Coverage   57.73%   57.75%   +0.01%     
==========================================
  Files         508      508              
  Lines       29287    29287              
  Branches     1381     1381              
==========================================
+ Hits        16909    16914       +5     
+ Misses      11170    11166       -4     
+ Partials     1208     1207       -1     

Impacted Files	Coverage Δ
webhook-service/handler/handler.go	86.20% <100.00%> (+2.95%)	⬆️
distributor/pkg/lib/controlplane/uniformlog.go	71.87% <0.00%> (-4.69%)	⬇️
shipyard-controller/handler/sequencedispatcher.go	69.62% <0.00%> (+1.48%)	⬆️

Flag	Coverage Δ
api	71.61% <ø> (ø)
approval-service	71.92% <ø> (ø)
bridge-server	58.30% <ø> (ø)
bridge2	73.13% <ø> (ø)
cli	43.15% <ø> (ø)
configuration-service	8.38% <ø> (ø)
distributor	72.14% <ø> (-0.28%)	⬇️
helm-service	49.23% <ø> (ø)
jmeter-service	13.88% <ø> (ø)
lighthouse-service	74.25% <ø> (ø)
mongodb-datastore	59.86% <ø> (ø)
remediation-service	79.77% <ø> (ø)
resource-service	82.85% <ø> (ø)
secret-service	79.25% <ø> (ø)
shipyard-controller	57.51% <ø> (+0.03%)	⬆️
statistics-service	59.96% <ø> (ø)
webhook-service	79.91% <100.00%> (+1.26%)	⬆️

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[RealAnna]

LGTM, failing integration tests are due to an old version