DOC-2425: Update migration guide and 7.0 release notes regarding force_hex_colour

[FarzadHayat]

Ticket: DOC-2425

Site: Migration guide Site: 7.0 Release Notes

Changes:

• Remove force_hex_color.adoc and its reference from content-formatting.adoc
• Update force_hex-color section in migration-from-6x.adoc
• Update force_hex-color section in 7.0-release-notes.adoc

Pre-checks:

• [x] Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/
• [-] modules/ROOT/nav.adoc has been updated (if applicable)
• [-] Files has been included where required (if applicable)
• [x] Files removed have been deleted, not just excluded from the build (if applicable)
• [-] Files added for New product features, and included a release note entry.

Review:

• [x] Documentation Team Lead has reviewed