emulators/qemu2: Mark DEPRECATED

• qemu 3.X branch in the tree seems like stable as no major complainnt has beeen reached and there has been no commit to 2.X ranch since last 13 months. 3.1.X will be updated shortly after build completes
• USES=gl is not always needed [1]

PR: 238364 [1]
Submitted by: pizzamig [1]