Commit fixes several ShellCheck found problems in Debian
Pre- and Postscripts.

Debian script mariadb-server-10.6.postrm contains shellcheck
Fixed problems are:
  https://www.shellcheck.net/wiki/SC2166 -- Prefer [ p ] && [ q ] as [ p -a q...
  https://www.shellcheck.net/wiki/SC2006 -- Use $(...) notation instead of le...
  https://www.shellcheck.net/wiki/SC1091 -- Not following: /usr/share/debconf...

Debian script mariadb-server-10.6.postinst contains shellcheck
Fixed problems are:
  https://www.shellcheck.net/wiki/SC2129 -- Consider using { cmd1; cmd2; } >>...
  https://www.shellcheck.net/wiki/SC1091 -- Not following: /usr/share/debconf...
  https://www.shellcheck.net/wiki/SC1072 -- Expected test to end here (don't ...
  https://www.shellcheck.net/wiki/SC1073 -- Couldn't parse this test expressi...
  https://www.shellcheck.net/wiki/SC1009 -- The mentioned syntax error was in...

Debian script mariadb-server-10.6.preinst contains shellcheck
Fixed problems are:
  https://www.shellcheck.net/wiki/SC2231 -- Quote expansions in this for loop...
  https://www.shellcheck.net/wiki/SC2166 -- Prefer [ p ] && [ q ] as [ p -a q...
  https://www.shellcheck.net/wiki/SC2001 -- See if you can use ${variable//se...
  https://www.shellcheck.net/wiki/SC1091 -- Not following: /usr/share/debconf...
  https://www.shellcheck.net/wiki/SC1007 -- Remove space after = if trying to...