feat: Update (and streamline) the default runtime to be DOTNET_8

[lachriz-aws]

The default runtime was set to DOTNET_6 while the docs said PROVIDED_AL2. This PR streamlines this, and changes the default runtime to be DOTNET_8.

[vlesierse]

With .NET 8 as default and the out of support of .NET 6 in November, this fix makes sense. It is a breaking change though, so we should mark it as features or include this in the other PR #259 with a minor version bump.

[lachriz-aws]

With .NET 8 as default and the out of support of .NET 6 in November, this fix makes sense. It is a breaking change though, so we should mark it as features or include this in the other PR #259 with a minor version bump.

Now that #259 is marked as a fix, I think it would make sense to follow your proposal of changing this PR (#260) to be a feature - and hence have them tracked as two different changes. I will change that now.

In terms of versioning - the current version is 0.0.4. Reading you suggestion to bump the minor version -- from a semantic versioning point of view, that would imply we do a 0.1.0 release, wouldn't it? Alternatively, we could also consider discussing if we instead want to do a first stable release (1.0.0) and have this PR included as part of that release?`

Many options to choose from - let me know what you think.

[vlesierse]

@lachriz-aws we can leave both PR separate. For versioning, currently this library is marked as experimental by the CDK team. We can take this to them on what is needed to bring it to a stable 1.0 release. Meanwhile following semantic versioning 0.1.0 would be a good next version for this feature.

[lachriz-aws]

Thanks for the above feedback @vlesierse. The PR now contains the following changes:

• The default runtime is now .NET 8.
• The lambda-handler-aot integ test project has been upgraded to .NET 8 (from .NET 7 which is end of life).
• The lambda-handler-docker integ test project has been upgraded to .NET 8 (from .NET 6).
• The lambda-handler integ test project has been upgraded to .NET 8 (from .NET 6).

Also, a new integ test project lambda-handler-dotnet6 has been added to test the use of a non-default .NET managed runtime (as per your feedback).

Finally, I should mention that the lambda-handler-aot integ test was failing when deployed from Windows x86_64 due to the following runtime identifier condition which caused the libicu dependency to not be bundled with the Lambda fn:

<ItemGroup Condition="'$(RuntimeIdentifier)' == 'linux-arm64'">
  <RuntimeHostConfigurationOption Include="System.Globalization.AppLocalIcu" Value="68.2.0.9" />
  <PackageReference Include="Microsoft.ICU.ICU4C.Runtime" Version="68.2.0.9" />
</ItemGroup>

Since globalization is not really core to the tests, I have simply replaced that inclusion with the below:

<!-- Disable globalization support to avoid having to take a dependency on libicu -->
<InvariantGlobalization>true</InvariantGlobalization>

[vlesierse]

LGTM