chore: upgrade templating engine to net8

[philasmar]

Issue #, if available:
DOTNET-8095

Description of changes:

• Upgrade templating engine from .NET5 to .NET8 since a lot of changes have happened to their public interfaces

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

Attention: Patch coverage is 86.95652% with 3 lines in your changes missing coverage. Please review.

Please upload report for BASE (feature/net8-upgrade@346b5c1). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/AWS.Deploy.Orchestration/TemplateEngine.cs	85.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@                   Coverage Diff                   @@
##             feature/net8-upgrade     #927   +/-   ##
=======================================================
  Coverage                        ?   62.13%           
=======================================================
  Files                           ?      294           
  Lines                           ?    10748           
  Branches                        ?     1602           
=======================================================
  Hits                            ?     6678           
  Misses                          ?     3482           
  Partials                        ?      588           

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