fix: replace instanceof check by Construct.isConstruct call

[swachter]

Reason for this change

Allow to use SelfManagedKafkaEventSource in symlinked setups

Description of changes

Replace instanceof check by Construct.isConstruct() call.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

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[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 1469935
• Result: SUCCEEDED
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[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.42%. Comparing base (24adca3) to head (1469935).
Report is 18 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #32353      +/-   ##
==========================================
- Coverage   78.46%   78.42%   -0.05%     
==========================================
  Files         106      106              
  Lines        7208     7208              
  Branches     1323     1323              
==========================================
- Hits         5656     5653       -3     
- Misses       1364     1368       +4     
+ Partials      188      187       -1     

Flag	Coverage Δ
suite.unit	78.42% <ø> (-0.05%)	⬇️

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Components	Coverage Δ
packages/aws-cdk	78.42% <ø> (-0.05%)	⬇️