xfsdump: update release.sh

Update release.sh script to automate releases and for-next updates even
more.

This is copy-paste from xfsprogs repository with a few changes:
- update shebang
- s/xfsprogs/xfsdump/
- remove old comment
- remind maintainer to upload tarball
- remove "The for-next branch has also ..." from for-next update email

Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>