Downgrade System.Memory to 4.5.4

[ViktorHofer]

Back the System.Memory update out (to unblock P6) as it's causing consumption issues in roslyn:

D:\a_work\1\s\src\roslyn\src\Features\Lsif\Generator\Microsoft.CodeAnalysis.LanguageServerIndexFormat.Generator.csproj : error NU1109: Detected package downgrade: System.Memory from 4.6.3 to centrally defined 4.5.5. Update the centrally managed package version to a higher version. [D:\a_work\1\s\src\roslyn\Roslyn.sln]
D:\a_work\1\s\src\roslyn\src\Features\Lsif\Generator\Microsoft.CodeAnalysis.LanguageServerIndexFormat.Generator.csproj : error NU1109: Microsoft.CodeAnalysis.Lsif.Generator -> System.CommandLine 2.0.0-ci -> System.Memory (>= 4.6.3) [D:\a_work\1\s\src\roslyn\Roslyn.sln]
D:\a_work\1\s\src\roslyn\src\Features\Lsif\Generator\Microsoft.CodeAnalysis.LanguageServerIndexFormat.Generator.csproj : error NU1109: Microsoft.CodeAnalysis.Lsif.Generator -> System.Memory (>= 4.5.5) [D:\a_work\1\s\src\roslyn\Roslyn.sln]
D:\a_work\1\s\src\roslyn\src\Workspaces\MSBuild\BuildHost\Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost.csproj : error NU1109: Detected package downgrade: System.Memory from 4.6.3 to centrally defined 4.5.5. Update the centrally managed package version to a higher version. [D:\a_work\1\s\src\roslyn\Roslyn.sln]
D:\a_work\1\s\src\roslyn\src\Workspaces\MSBuild\BuildHost\Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost.csproj : error NU1109: Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost.Symbols -> System.CommandLine 2.0.0-ci -> System.Memory (>= 4.6.3) [D:\a_work\1\s\src\roslyn\Roslyn.sln]
D:\a_work\1\s\src\roslyn\src\Workspaces\MSBuild\BuildHost\Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost.csproj : error NU1109: Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost.Symbols -> System.Memory (>= 4.5.5) [D:\a_work\1\s\src\roslyn\Roslyn.sln]