User can get stuck with sending read receipts disabled if server stopped supporting it

[SimonBrandner]

• Use a server that supports MSC2285
• Disable sending read receipts
• The server stops supporting MSC2285
• You're stuck with no RRs and no way to clear notifs

It's an edge-case but we should at least provide a way to enable them again

Spotted in #23433

[SimonBrandner]

For those affected by this (cc @Flupp), you should be able to open the console (Ctrl+Shift+I) and input mxSettingsStore.setValue("sendReadReceipts", null, "account", true) which should enable sending read receipts again