AddBackticksToIdentifierIfNeeded adds ticks for `not` word.

Please provide a succinct description of the issue.

**Repro steps**

Local fsi:

```fsharp
#r @"C:\Users\fverdonck\Projects\fsharp\artifacts\bin\FSharp.Core\Debug\netstandard2.0\FSharp.Core.dll"
#r @"C:\Users\fverdonck\Projects\fsharp\artifacts\bin\FSharp.Compiler.Service\Debug\netstandard2.0\FSharp.Compiler.Service.dll"

FSharp.Compiler.Syntax.PrettyNaming.AddBackticksToIdentifierIfNeeded "not"
|> printfn "FSharp.Compiler.Syntax.PrettyNaming.AddBackticksToIdentifierIfNeeded : %s"
```

> dotnet artifacts\bin\fsi\Debug\net5.0\fsi.dll C:\Users\fverdonck\Projects\scripts\fsharp\ast-from-local-compiler.fsx

**Expected behavior**

In previews version of FCS, no ticks are added.

**Actual behavior**

``not``

**Known workarounds**

Provide a description of any known workarounds.

**Related information**

Provide any related information (optional):

FCS 41.0.0-preview.21472.3