Raise minimum VS Code API version to 1.51 (to match LSP minimum)

[jakebailey]

Through some recent reports and stack traces, I've noticed that the LSP client enforces its own (sort of undocumented) minimum VS Code API version (see: https://github.com/microsoft/vscode-languageserver-node/blob/20681d7632bb129def0c751be73cf76bd01f2f3a/client/src/node/main.ts#L25). This results in crashes when someone's using an LS but a version below their minimum.

Right now, the client we use (which is a prerelease) has a minimum of 1.51 to start a client. I think it might be a good idea to boost the required version up to that level to prevent oddities. Eventually, it'll be somewhat required when jedi is behind an LS.

Eventually, I'll want to bump the LSP library to the latest stable, but that bumps the minimum to 1.52.

Some examples:

• something went wrong activating IntelliCode support for Python+ Error while activating Python: {} MicrosoftDocs/intellicode#266
• https://stackoverflow.com/questions/65301266/error-loading-python-for-vs-code-intellicode