Add note in upgrade guide to choose the correct version tag

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/3448

@rspeicher I'm assigning to you since as a release manager you should know better. Let's keep it in this guide for now and then start copying it in the future ones.

as a release manager you should know better.

What do you mean?

@axil Ping.

Sorry, I meant that since you are the one to make sure the upgrade guides are present in each release, you should know better if this text needs to be there. Not the wiser choice of words...

@axil Ok. It read a little sassy but I didn't think you meant it that way.

@axil Is this ready?

Milestone changed to 8.6

@axil Could you add this note to the more recent upgrade guides as well (we'll automate the creation of upgrade guides later but for now I'd say let's add this text manually)?

Added 3306 commits:

• 11dda8db...dadd28e3 - 3304 commits from branch master
• 4d3e8cee - Merge branch 'master' into docs_select_version_to_install
• e1ce7a47 - Add note in rest of update guides

Reassigned to @axil

mentioned in commit e877cfda

Status changed to merged

Added ~149423 label

@rymai done!

@axil Awesome, thanks!

Picked into 8-6-stable, will go into 8.6.0-rc5.

Removed ~149423 label

mentioned in commit dc0b12dc