scripts/installer.sh: add FreeBSD 15 (#16741)

* Update installer.sh add FreeBSD ver 15

this should fix the issue on https://github.com/tailscale/tailscale/issues/16740

Signed-off-by: TheBigBear <471105+TheBigBear@users.noreply.github.com>

* scripts/installer.sh: small indentation change

Signed-off-by: Erisa A <erisa@tailscale.com>
Fixes #16740

---------

Signed-off-by: TheBigBear <471105+TheBigBear@users.noreply.github.com>
Signed-off-by: Erisa A <erisa@tailscale.com>
Co-authored-by: Erisa A <erisa@tailscale.com>

scripts/installer.sh

@@ -408,7 +408,8 @@ main() {
 		freebsd)
 			if [ "$VERSION" != "12" ] && \
 			   [ "$VERSION" != "13" ] && \
-			   [ "$VERSION" != "14" ]
+			   [ "$VERSION" != "14" ] && \
+			   [ "$VERSION" != "15" ]
 			then
 				OS_UNSUPPORTED=1
 			fi