MDEV-28605: Change wrong plugin config installation location (#2160)

Preset include directory for configuration files below MariaDB 10.5 is
/etc/mysql/conf.d

Change installation location wrong plugin installation location from
/etc/mysql/mariadb.d to default include directory /etc/mysql/conf.d.
Change makes gssapi-server, oqgraph, rocksdb and tokudb plugins
loading work after installation

NOTE TO MERGERS: This commit should be upstream to MariaDB 10.4 only!
Merging to MariaDB 10.5 and above leads to major problems.

debian/mariadb-plugin-gssapi-server.install

-etc/mysql/conf.d/auth_gssapi.cnf etc/mysql/mariadb.conf.d
+etc/mysql/conf.d/auth_gssapi.cnf
 usr/lib/mysql/plugin/auth_gssapi.so

debian/mariadb-plugin-oqgraph.install

-etc/mysql/conf.d/oqgraph.cnf etc/mysql/mariadb.conf.d
+etc/mysql/conf.d/oqgraph.cnf
 usr/lib/mysql/plugin/ha_oqgraph.so

debian/mariadb-plugin-rocksdb.install

-etc/mysql/conf.d/rocksdb.cnf etc/mysql/mariadb.conf.d
+etc/mysql/conf.d/rocksdb.cnf
 usr/bin/myrocks_hotbackup
 usr/bin/mysql_ldb
 usr/bin/sst_dump

debian/mariadb-plugin-tokudb.install

-etc/mysql/conf.d/tokudb.cnf etc/mysql/mariadb.conf.d
+etc/mysql/conf.d/tokudb.cnf
 etc/systemd/system/mariadb.service.d/tokudb.conf
 usr/bin/tokuft_logprint
 usr/bin/tokuftdump