Prepare 6.1.0-rc1 release

Prepare 6.1.0-rc1 release

changelog: explicitly link to graphql-php release

which documents the breaking changes in detail

Prepare 5.1.5 release

Merge pull request rebing#692 from mfn/mfn-merge-5.x

[6.x] Merge 5.x into master/6.x

Prepare for 5.1.5-rc1

changelog: fix typo

changelog: prepare for 5.1.3 release

changelog: fix dates 🤷‍♀️

Prepare next release