Update Soroban-RPC specfile

[ElliotFriend]

The open-rpc specfile we're using to generate the documentation has fallen out of date. It's currently v20.1.0 while the released software is v20.3.1. The specfile (and the produced documentation, therefore) should be updated to the most current version.

[ElliotFriend]

From what I can tell, most of the correct types and stuff will be found in this directory: https://github.com/stellar/soroban-rpc/tree/main/cmd/soroban-rpc/internal/methods

[briwylde08]

Related: #949

[ElliotFriend]

The spec has long since moved past the version this issue flagged — it now includes getLedgers, getTransactions, getVersionInfo and the rest of the v21/v22-era surface; ongoing spec sync is tracked in #949. The one leftover (info.version still literally reads "20.1.0") is captured in #2581. Closing as completed.