[BUG]: `.gitmodules` content  is outdated and incorrect

## Bug Report

**Current Behavior**
The content in `gitmodules` needs to be updated. `dpdk` and `pktgen-dpdk` have currently replaced the master branch with the main branch, and the version corresponding to the dpdk commit id in gitmodules is 18.11, not 20.05

**Environment**
- OS:  ubuntu20.04
- onvm version:  current master branch version


