Links between migration docs are out of date

[sjohnr]

The links between 5.8 and 6.0 migration docs point to a specific version (5.8.0 and 6.0.0 respectively). We should link to the latest in the release line.

[sjohnr]

Note: It does not currently appear to be possible to link to a release line version in Antora (e.g. 5.8), only a specific component version (e.g. 5.8.2). Since we are publishing docs based on tags, the component version will change on each release. For now, I'll carry this issue forward to each point release as a type: reminder.