netbird: update to 0.21.7
1. Release notes:
https://github.com/netbirdio/netbird/releases/tag/v0.21.2
https://github.com/netbirdio/netbird/releases/tag/v0.21.3
https://github.com/netbirdio/netbird/releases/tag/v0.21.4
https://github.com/netbirdio/netbird/releases/tag/v0.21.5
https://github.com/netbirdio/netbird/releases/tag/v0.21.6
https://github.com/netbirdio/netbird/releases/tag/v0.21.7
2. Update GO_PKG_LDFLAGS, because of https://github.com/netbirdio/netbird/commit/
292ee260ad564d1e65199b1cb3430b0cd7ba9646
3. Define the configuration file.
Signed-off-by: Salem Huang <solohoh@hotmail.com>