Deprecate `Parser::setCharset/getCharset`

[oliverklee]

Deprecate it in 8.x and remove it in 9.0.

[JakeQZ]

Shouldn't we deprecate it in 9.0 and remove it in 10.0? 8.x is already released.

[oliverklee]

Once merged, I'd like to backport this deprecation to 8.x so the deprecation annotation will be in the hopefully-to-be-released-soon 8.7, which would allow us to remove the deprecated things in 9.0.

I'd also like to backport the other deprecations if possible (#696) for the same reason.

[JakeQZ]

I'd like to backport this deprecation to 8.x so the deprecation annotation will be in the hopefully-to-be-released-soon 8.7, which would allow us to remove the deprecated things in 9.0.

OK. This is consistent with Semantic Versioning