Clean up handling of --enable-ftp (docker-library#1491)

* Drop PHP 8.2.14 / 8.3.1 for `--enable-ftp`

These versions have been superseeded by newer versions and thus the condition
will no longer match.

see 641cf9a

* Add `--enable-ftp` for future 8.1.x versions

8.1.x is security-only and future releases happen on a case to case basis to
fix security issues only. We likely should not break the build for those folks
still on 8.1.

see 641cf9a

Dockerfile-linux.template

@@ -318,7 +318,7 @@ RUN set -eux; \
 # https://github.com/docker-library/php/issues/822
 		--with-pic \
 		\
-{{ if .version as $v | [ "8.1.27", "8.2.14", "8.3.1" ] | index($v) then ( -}}
+{{ if [ "8.1" ] | index(env.version | rtrimstr("-rc")) then ( -}}
 # --enable-ftp is included here for compatibility with existing versions. ftp_ssl_connect() needed ftp to be compiled statically before PHP 7.0 (see https://github.com/docker-library/php/issues/236).
 		--enable-ftp \
 {{ ) else "" end -}}