CMP0022: Fix version documented to support LINK_PUBLIC/LINK_PRIVATE

These have been available since 2.8.7, not 2.8.9 as the docs said.
Merge branch 'doc-CMP0022-compat-version-typo' into release
Merge topic 'doc-CMP0022-compat-version-typo'

91857e04 CMP0022: Fix version documented to support LINK_PUBLIC/LINK_PRIVATE
Merge branch 'release'