docs: Amend RPC Transaction History proposal (#20794) (#20812)

# Problem

The initial proposal ruled out implementing BigTable queries for
the `getBlockTime` RPC, but then it was implemented a couple months
later. Indicating that the functionality was never implemented in
the "implemented-proposals" document is a little confusing, so let's
bring the document in line with what actually happened. 🦾

# Summary of Changes

Remove the blurb about how `getBlockTime` was going to be deprecated
and add it to the list of calls that didn't yet support BigTable
queries at the time the proposal was written.

(cherry picked from commit 0c7bade0b2)

Co-authored-by: Arthur Burkart <arthur@presynce.com>

docs/src/implemented-proposals/rpc-transaction-history.md

@@ -16,10 +16,7 @@ The affected RPC endpoints are:
 - [getConfirmedSignaturesForAddress](developing/clients/jsonrpc-api.md#getconfirmedsignaturesforaddress)
 - [getConfirmedTransaction](developing/clients/jsonrpc-api.md#getconfirmedtransaction)
 - [getSignatureStatuses](developing/clients/jsonrpc-api.md#getsignaturestatuses)
-
-Note that [getBlockTime](developing/clients/jsonrpc-api.md#getblocktime)
-is not supported, as once https://github.com/solana-labs/solana/issues/10089 is
-fixed then `getBlockTime` can be removed.
+- [getBlockTime](developing/clients/jsonrpc-api.md#getblocktime)
 
 Some system design constraints: