Produce a warning for deprecated features related to JSON outputs

[wenzeslaus]

This issue is to track deprecation warnings for version 8.6 (next minor release).

After addition of format=json, format=csv, or others to tools producing text on standard output, some parameters, especially usage -g in many tools, are deprecated.

Usually, the deprecation warning was implemented as a verbose message to allow people adopt the new behavior in v8.5 while not being intrusive with a warning when they use something which was the only possible way in the previous version (8.4). In version 8.6 (or possibly a later version), we should change this to a warning to strongly encourage the change.

In the code, we may have something like this:

G_verbose_message(
            _("Flag 'g' is deprecated and will be removed in a future "
              "release. Please use format=csv instead."));

And that should now change to this:

G_warning(
            _("Flag 'g' is deprecated and will be removed in a future "
              "release. Please use format=csv instead."));

This will apply to multiple tools and a full text search of [deprecated] should generally be sufficient to identify the places which needs a change.

In addition to the message, the documentation may to be updated if it specifically mentions a version or the message, so a documentation review is needed. The following example of documentation would not need any change:

The -g flag is deprecated and will be removed in a future release. Please use format=csv option with the -p flag instead.

The parameters should also be reviewed, but generally should not need any change just like the doc. An example which does not need a change:

    csv_print = G_define_flag();
    csv_print->key = 'g';
    csv_print->label = _(
        "Print all map connection parameters in a legacy format [deprecated]");

This can in theory be applied to version 9.0 without breaking semantic versioning expectations because the features would be already deprecated at the time of the release.