chore: add jdk 21 job for showcase tests

[mpeddada1]

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Fixes #<issue_number_goes_here> ☕️

[mpeddada1]

ci/ showcase (21)

Caused by: com.google.api.client.http.HttpResponseException: 400 Bad Request
POST http://localhost:7469/v1beta1/repeat:body
{"error":{"code":400,"message":"error reading body params '*': proto: syntax error (line 1:513): invalid value \u0000","details":null,"Body":"","Header":null,"Errors":null}}
	at com.google.api.client.http.HttpResponseException$Builder.build(HttpResponseException.java:293)
	at com.google.api.client.http.HttpRequest.execute(HttpRequest.java:1118)
	at com.google.api.gax.httpjson.HttpRequestRunnable.run(HttpRequestRunnable.java:115)
	... 6 more

[sonarcloud]

Quality Gate passed for 'gapic-generator-java-root'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[sonarcloud]

Quality Gate passed for 'java_showcase_integration_tests'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud