Bug#1073102: rtpengine: replace to-be-removed markdown build-dependency

[Chris Hofstaedtler]

Source: rtpengine
Version: 11.5.1.24-1
Severity: serious
Control: block 1072958 by -1

Your package build-depends on markdown. Per bug #1063645, markdown is not
maintained upstream or in Debian and should be removed.
Drop-in alternatives, for examples the suggested `discount` or
`python3-markdown` or `libtext-markdown-perl`.

`discount` and `libtext-markdown-perl` provide a `markdown` program if your
package needs that.