Document webpack change: Remove concord support

[webpack-bot]

A pull request by @ooflorent was merged and maintainers requested a documentation change.

See pull request: webpack/webpack#8436

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What kind of change does this PR introduce?

refactoring

Did you add tests for your changes?

no

Does this PR introduce a breaking change?

yes

What needs to be documented once your changes are merged?

resolve.concord no longer exists

[EugeneHlushko]

This doesnt currently exist in the docs so should only be used in migration guide note i guess.