Update doctrine/collections requirement from ^1.6 to ^2.0

[dependabot[bot]]

Updates the requirements on doctrine/collections to permit the latest version.

Release notes

Sourced from doctrine/collections's releases.

2.0.1

Release Notes for 2.0.1

2.0.x bugfix release (patch)

2.0.1

• Total issues resolved: 0
• Total pull requests resolved: 2
• Total contributors: 2

Bug,Static Analysis

• 340: Mark closure arguments as always-provided thanks to @​greg0ire

Documentation

• 338: Fix markup thanks to @​xepozz

Commits

• d766a45 Merge pull request #340 from greg0ire/no-optional
• 1ca4949 Mark closure arguments as always-provided
• 86a79ea Merge pull request #339 from greg0ire/shallow-tree
• b74b686 Merge pull request #338 from xepozz/patch-1
• cc369aa Fix markup
• e1751ba Use a shallower tree
• 6ca4391 Merge pull request #335 from greg0ire/update-branch-metadata
• 14752f3 Merge pull request #336 from greg0ire/fix-upgrade
• 5a55b9b Improve formatting
• f1902b2 Document recommended changes
• Additional commits viewable in compare view

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[what-the-diff[bot]]

• The composer.json file was changed
• In the require section, doctrine/collections version number has been updated from 1.6 to 2.0

[roxblnfk]

Depends on https://github.com/cycle/orm/issues/376