[tidb/master] migrate pull_mysql_test_next_gen to GCP

[dillon-zheng]

Parent issue: #4393

Target:

• job: pingcap/tidb/pull_mysql_test_next_gen
• branch: master

Status from Feishu:

• source: https://pingcap.feishu.cn/wiki/ZEqcwcBRoihglDkv167cHR3ynWf
• status: N/A
• last success: 2025-07-29

Acceptance:

• job has been validated on GCP
• migration config has been merged
• result has been recorded in migration notes