Link checker fails due to 429 Too Many requests

[sverhoeven]

The changelog contains a lot of links to github. Running link checker in quick succession will cause 429 Too Many requests errors.
This causes CI jobs to be red.

Possible solution could be to add 'retryOn429: true' to mlc config, see https://github.com/tcort/markdown-link-check#markdownlinkcheckmarkdown-opts-callback

[egpbos]

Was fixed in #307.