Replace extern with external in interop error messages

[gabilang]

Purpose

$title

Fixes #41620

Approach

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Samples

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Remarks

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Check List

• Read the Contributing Guide
• Updated Change Log
• Checked Tooling Support (#)
• Added necessary tests
  • Unit Tests
  • Spec Conformance Tests
  • Integration Tests
  • Ballerina By Example Tests
• Increased Test Coverage
• Added necessary documentation
  • API documentation
  • Module documentation in Module.md files
  • Ballerina By Examples

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1a16695) 76.67% compared to head (94e1a1a) 76.68%.
Report is 100 commits behind head on master.

❗ Current head 94e1a1a differs from pull request most recent head 1af70be. Consider uploading reports for the commit 1af70be to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #41632      +/-   ##
============================================
+ Coverage     76.67%   76.68%   +0.01%     
- Complexity    52669    52676       +7     
============================================
  Files          2881     2881              
  Lines        198686   198686              
  Branches      25822    25822              
============================================
+ Hits         152350   152371      +21     
+ Misses        37896    37880      -16     
+ Partials       8440     8435       -5     

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[gabilang]

Closing this PR since the old-style external function wrapper related changes are removed with the PR #41658