chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vendor/golang.org/x/sys/unix/ifreq_linux.go

@@ -111,9 +111,7 @@ func (ifr *Ifreq) SetUint32(v uint32) {
 // clear zeroes the ifreq's union field to prevent trailing garbage data from
 // being sent to the kernel if an ifreq is reused.
 func (ifr *Ifreq) clear() {
-	for i := range ifr.raw.Ifru {
-		ifr.raw.Ifru[i] = 0
-	}
+	clear(ifr.raw.Ifru[:])
 }
 
 // TODO(mdlayher): export as IfreqData? For now we can provide helpers such as