update version in installation doc
update VERSION to 6.8.0.rc1
add 67-68 update guide
bump versions in update guide
Merge branch 'rc1-updates' into 'master'

Rc1 updates

#1178

Please check it well :smiley:

If this gets merged, the annotated version tag 6.8.0.rc1 will point to https://dev.gitlab.org/gitlab/gitlabhq/commit/e6b668ab0169f981a5c4162cd8625daf02e6f5a8

/cc @sytse @jacobvosmaer @marin