Update the documentation regarding deprecated features planned to be removed and new keywords introduced in v0.9

[matheusaaguiar]

There are features marked as deprecated and planned to be removed, many of them in the next breaking release v.0.9, which are listed in #15795 . We also need to document the promotion of many identifiers to keywords, as listed in #14770.

While some of the necessary modifications are already done during the PR to effectively deprecate the features and add the new keywords, the documentation and included examples should be revised and adjusted accordingly to reflect the changes. A new breaking changes page for v.0.9 should be added to track the removed features and introduced keywords, as well as any other syntactic or semantic change.