Added modified region tag for migration

[gotochkin]

Description

The region tag custom_attribute_filter_multiattributes in the file jobs/v3/src/main/java/com/google/samples/CustomAttributeSample.java is being migrated adding a correct "job" prefix to the region. After merging the old region will be deprecated and removed from the documents.

Internal: b/347826138

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• [x] I have followed Sample Format Guide
• [ ] pom.xml parent set to latest shared-configuration
• [ ] Appropriate changes to README are included in PR
• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [x] Tests pass: mvn clean verify required
• [x] Lint passes: mvn -P lint checkstyle:check required
• [ ] Static Analysis: mvn -P lint clean compile pmd:cpd-check spotbugs:check advisory only
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [ ] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• [x] Please merge this PR for me once it is approved

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 1 region tag.

- [jobs/v3/src/main/java/com/google/samples/CustomAttributeSample.java:150](https://github.com/gotochkin/java-docs-samples/blob/11798a3603a0e5237567b36d7b1a50a076efcbd2/jobs/v3/src/main/java/com/google/samples/CustomAttributeSample.java#L150), tag `job_custom_attribute_filter_multi_attributes`

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