Fix casing for event key value

[MarcoRossignoli]

This pull request includes a change to the JsonRpcStrings class in the JsonRpcMethods.cs file to standardize the casing of the EventName constant.

Standardization of constant naming:

• src/Platform/Microsoft.Testing.Platform/ServerMode/JsonRpc/JsonRpcMethods.cs: Changed the constant EventName to eventName to maintain consistent casing.

cc: @drognanar

[Evangelink]

Shall we backport this to 1.4.2? Otherwise we will only start collecting telemetry from the next release (in some time).

cc @pavelhorak

[pavelhorak]

Shall we backport this to 1.4.2? Otherwise we will only start collecting telemetry from the next release (in some time).

cc @pavelhorak

I'd prefer so