clientupdate: handle multiple versions in "apk info tailscale" output (#11310)

The package info output can list multiple package versions, and not in
descending order. Find the newest version in the output, instead of the
first one.

Fixes #11309

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

clientupdate/clientupdate_test.go

@@ -251,6 +251,29 @@ tailscale installed size:
 			out:     "",
 			wantErr: true,
 		},
+		{
+			desc: "multiple versions",
+			out: `
+tailscale-1.54.1-r0 description:
+The easiest, most secure way to use WireGuard and 2FA
+
+tailscale-1.54.1-r0 webpage:
+https://tailscale.com/
+
+tailscale-1.54.1-r0 installed size:
+34 MiB
+
+tailscale-1.58.2-r0 description:
+The easiest, most secure way to use WireGuard and 2FA
+
+tailscale-1.58.2-r0 webpage:
+https://tailscale.com/
+
+tailscale-1.58.2-r0 installed size:
+35 MiB
+`,
+			want: "1.58.2",
+		},
 	}
 
 	for _, tt := range tests {