Resolve "Follow-up from "Upgrade to Git 2.26""

What does this Merge Request do?

It updates EXPECTED_GIT_VERSION to 2.26 in lib/gdk/dependencies.rb as this is now the default.

This should have been done in !1145 (merged) but was forgotten.

Merge Request checklist

• No new tests seem to be needed.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation was updated in: !1145 (merged)
• This is a gdk doctor thing so no need to add tests in it.

Closes #847 (closed)