Cleanup `NET8_0_OR_GREATER` symbol

[xtqqczze]

Since https://github.com/dotnet/runtime/pull/90880, net8.0 has been the minimum.

For files that are compiled for other platforms (.NET Standard, .NET Framework) NET8_0_OR_GREATER can be replaced with NET.

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[deeprobin]

fyi: Current usages of NET8_0_OR_GREATER

$ git grep "NET8_0_OR_GREATER" | \
> awk -F":" '{print $1}' | \
> awk -F"/" '{print $1"/"$2"/"$3}' | \
> sort | uniq -c | \
> sort -nr
     36 src/libraries/System.Text.Json
     27 src/libraries/System.DirectoryServices
     20 src/libraries/System.Configuration.ConfigurationManager
     18 src/libraries/System.DirectoryServices.AccountManagement
     14 src/libraries/Common
     13 src/libraries/Microsoft.Extensions.Options
     12 src/libraries/System.Management
     12 src/libraries/System.DirectoryServices.Protocols
     12 src/libraries/System.Diagnostics.EventLog
     10 src/libraries/System.Diagnostics.DiagnosticSource
     10 src/libraries/Microsoft.Bcl.TimeProvider
      9 src/libraries/System.Reflection.Metadata
      8 src/libraries/System.Security.Permissions
      8 src/libraries/System.ComponentModel.Composition
      5 src/libraries/System.Security.Cryptography.Pkcs
      5 src/libraries/System.Reflection.MetadataLoadContext
      5 src/libraries/System.Collections.Immutable
      5 src/libraries/Microsoft.Extensions.Logging.Abstractions
      4 src/libraries/System.ServiceModel.Syndication
      4 src/libraries/System.IO.Packaging
      4 src/libraries/System.Formats.Cbor
      4 src/libraries/Microsoft.Extensions.Http
      3 src/libraries/System.Text.RegularExpressions
      3 src/libraries/System.Private.CoreLib
      3 src/libraries/System.Formats.Asn1
      3 src/libraries/System.Data.Odbc
      3 src/libraries/Microsoft.Extensions.Hosting.Abstractions
      2 src/tasks/Common
      2 src/libraries/System.Windows.Extensions
      2 src/libraries/System.Threading.Tasks.Dataflow
      2 src/libraries/System.Threading.Channels
      2 src/libraries/System.ServiceProcess.ServiceController
      2 src/libraries/System.Runtime.Serialization.Xml
      2 src/libraries/System.IO.Compression
      2 src/libraries/System.Formats.Nrbf
      2 src/libraries/System.Data.OleDb
      2 src/libraries/System.Collections
      1 src/libraries/System.Threading.RateLimiting
      1 src/libraries/System.Speech
      1 src/libraries/System.Security.Cryptography.Xml
      1 src/libraries/System.Reflection.Context
      1 src/libraries/System.Net.Http.WinHttpHandler
      1 src/libraries/Microsoft.Extensions.FileProviders.Physical
      1 src/libraries/Microsoft.Bcl.Cryptography