#39072

[ci skip]

This commit extracts `kubectl`, `helm` and `codeclimate` versions as CI
variables.

`kubectl` changes from latest stable version to `1.8.6`, the other two are just
extracted in order to be easily updated; now we can also test tool upgrades
overriding CI secret variables.

The st_commits and st_diffs columns on merge_request_diffs historically held the
YAML-serialised data for a merge request diff, in a variety of formats.

Since 9.5, these have been migrated in the background to two new tables:
merge_request_diff_commits and merge_request_diff_files. That has the advantage
that we can actually query the data (for instance, to find out how many commits
we've stored), and that it can't be in a variety of formats, but must match the
new schema.

This is the final step of that journey, where we drop those columns and remove
all references to them. This is a breaking change to the importer, because we
can no longer import diffs created in the old format, and we cannot guarantee
the export will be in the new format unless it was generated after this commit.

Signed-off-by: Rémy Coutable <remy@rymai.me>

Followed the command from
https://gitlab.com/gitlab-org/release-tools/blob/master/doc/release-candidates.md#step-4-update-the-dependencies-license-list

    bin/bundle exec license_finder report --format=csv
--save=vendor/licenses.csv

    bin/rake gitlab:update_templates