minor change in the Debian Policy for Ada

[Nicolas Boulenguez <nicolas@debian.org>]

Hello.

During the gnat-10 transition, the release team suggested a change in
order to help the automatic migration from unstable to testing of all
Ada packages at once.
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=975589#24

Currently, the policy requires that
  Package: lib...-dev
  Depends: gnat, gnat-X
The suggestion is to replace this with:
  Depends: gnat (>= X), gnat (<< X+1)

This seems equivalent.
Unless someone objects, I suggest to update the policy.

Then, I intend to try the following changes in experimental.
* libxmlada gprbuild: change ada:Depends in debian/rules
* dh-ada-library: change ada:Depends in src/dh_ada_library.adb.
  Let’s say the new version is 7.5.

If all tests pass, the following changes seem trivial enough to be
part of the collective reupload to unstable.
* adasockets plplpto: manually edit the dependency
* for each library that Build-Depends: dh-sequence-ada-library,
  simply add            Build-Depends: dh-ada-library (>= 7.5).