Deprecate `setColour`

[BeksOmega]

Deprecate the setColour method on the block, with a message to use CSS instead.

[ga-fleury]

@BeksOmega is there a deletion timeframe for this method? (should I assume v13?)

[BeksOmega]

We're not exactly sure yet! That's why it's not marked for help wanted or good first issue at the moment.

[BHAV0207]

hey @BeksOmega I would like to work on this issue please assign it to me

[Spoorthy1423]

I would like to work on this issue, please assign this issue to me @BeksOmega