Limit ol-ext version to 4.0.18 and update yarn.lock

[sanak]

Fixes the following error.
https://github.com/sanak/redmine_gtt/actions/runs/10280544083/job/28448110645#step:8:53

ERROR in ./node_modules/ol-ext/interaction/Hover.js 2:0-45
Module not found: Error: Can't resolve '../util/element' in '/__w/redmine_gtt/redmine_gtt/redmine/plugins/redmine_gtt/node_modules/ol-ext/interaction'
Did you mean 'element.js'?
BREAKING CHANGE: The request '../util/element' failed to resolve only because it was resolved as fully specified

Changes proposed in this pull request:

• To avoid above error for CI test before PR:Updates OpenLayers to v10 #330 will be merged.

@dkastl
I think that you are waiting for ol-ext next release (4.0.22) which includes your PR, but now CI is failing (like on my fork action), so this PR is just limit ol-ext version to 4.0.18 like your PR:#330.
Could you check this when you have a time ?

After this PR is approved and merged, I will create PR for deface override fix.

[dkastl]

You're right, I've been waiting for a new release, because they usually come quickly ... except this time, it seems. So this PR is a good idea.