add git parsing to Rakefile to determine future_version

[rwaffen]

do as we do it in pdc and add git parsing to determine future_version from the git release branch name.

release branch has to be named release-vx.y.z and vx.y.z will be parsed and added on thy fly in the rakefile

see
https://github.com/betadots/pdc/blob/30b4115698d5b175437043a1efc70fefbec22ce8/Rakefile#L15

update

hdm/RELEASE_howto.md

Lines 16 to 28 in 33e29bc

	## Create Release PR
	```shell
	export CHANGELOG_GITHUB_TOKEN="github_TOKEN"
	git switch -c release/X.Y.Z
	```
	edit `Rakefile`, search replace `future_release`
	```text
	config.future_release = 'X.Y.Z'
	```

[rwaffen]

done with #308