VERSION: debian compliant version string

https://www.debian.org/doc/debian-policy/ch-controlfields.html#version

The upstream_version must contain only alphanumerics 6 and the
characters . + - ~ (full stop, plus, hyphen, tilde) and should start
with a digit.`

If there is no debian_revision (which there isn't) then hyphens are
not allowed.

VERSION

 MYSQL_VERSION_MAJOR=11
 MYSQL_VERSION_MINOR=6
 MYSQL_VERSION_PATCH=0
-MYSQL_VERSION_EXTRA=_vector
+MYSQL_VERSION_EXTRA=vector
 SERVER_MATURITY=alpha