DOC Add notice regarding session renewal API changes

[Cheddam]

Description

Adds a note to the 6.0.0 release doc regarding the session renewal API changes.

Issues

• Remember Me functionality fails when renewal is interrupted silverstripe-framework#11281

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[GuySartorelli]

LGTM, will merge after code PR

[GuySartorelli]

Retargetted to 6.0 and rebased