Remove rspack.core <1.6.0 constraint

[Pijukatel]

Currently updating rspack to the newly released version 1.6.0 breaks the docs build and so the version has been pinned to <1.6.0.

Delete this constraint once the upstream repos are compatible with rspack 1.6.0

[vdusek]

once the upstream issues are resolved, remove this constraint in client and crawlee as well

[janbuchar]

@Pijukatel @vdusek is there an upstream issue that you could link here so that we can easily check if the problem still exists?

[Pijukatel]

I failed to identify where the problem is. I guessed that it was facebook/docusaurus#11529, but their CI passed with such a bump, so it was probably something else.

I am planning just to watch the bumps and relax the constraint once some new passes. Recent one failed for example:
#722