feat: Mark `getDefaultEndpoint()` with @ObsoleteApi

[lqiu96]

The endpoint is not longer set with the default endpoint. The EndpointContext will be able resolve the client's endpoint

[lqiu96]

Removing the getDefaultEndpoint() from being set in this PR: https://github.com/googleapis/sdk-platform-java/pull/2348

[sonarqubecloud[bot]]

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

Failed conditions
30.0% Duplication on New Code (required ≤ 3%)

See analysis details on SonarCloud

[sonarqubecloud[bot]]

Quality Gate passed for 'java_showcase_integration_tests'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
50.0% Duplication on New Code

See analysis details on SonarCloud