Add deprecated flag to RPCs

[jacogr]

Closes #5241

Adds the @deprecated flag to both the metadata generation as well as the TS interfaces.

However with TS augmentation this is not prefect and there is (currently) no way to make it perfect, the levers that we are able to pull is correct for the specific generation, aka

      /**
       * @deprecated Use `api.rpc.state.getKeysPaged` to retrieve keys
       * Returns the keys with prefix, leave empty to get all the keys (deprecated: Use getKeysPaged)
       **/
      getPairs: AugmentedRpc<(prefix: StorageKey | string | Uint8Array | any, at?: BlockHash | string | Uint8Array) => Observable<Vec<KeyValue>>>;

As an example of non-perfectness in VSCode -

However when going to the specific endpoint, it does highlight as deprecated -

As well as -

The first one should mark as striked in the editor as well as having the correct bubble as per the last 2.

[polkadot-js-bot]

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