SE.Redis DC backend / HybridCache : add net8.0 TFM

[mgravell]

Context: Aspire has Garnet support via AddGarnet(); Aspire targets LTS, so 8.0; we would like Aspire's Garnet support to work for more than just the raw IConnectionMultiplexer usage; specifically, we would like at least some of the inbuilt cache implementations to work. Prior to 9.0, the redis-based IDistributedCache does not work with Garnet, because it uses Lua, which Garnet does not support. It is not currently planned to backport the necessary Garnet support to 8.0.

In theory, the pre-existing NS targets on these libraries means that 8.0 apps can consume the 9.0 OOB package and work, but we have been advised to avoid the NS target here, as crossing the streams may cause some assembly load problems at runtime. The suggestion is to add an 8.0 TFM to the 9.0 build, which is unusual but should be valid. In particular, this applies to:

1. Microsoft.Extensions.Caching.StackExchangeRedis, which adds Garnet support by switching from Lua to direct commands
2. Microsoft.Extensions.Caching.Hybrid, new feature in 9.0

(note that the "runtime" piece i.e. Microsoft.Extensions.Caching.Abstractions already has an explicit 8.0 TFM in the 9.0 build)

Both of these already have down-level targets, so this is not "new", just a new explicit TFM in the existing matrix.

To avoid leaving 8.0 in hard-to-find places, I have also introduced $(CurrentLtsTargetFramework) via eng/Versions.props; I have validated that this continues to work correctly when it matches the $(DefaultNetCoreTargetFramework)

/cc @mitchdenny

[mgravell]

probably also /cc @wtgodbe in case this is a "Known Bad Thing™, verboten"

[halter73]

Seems reasonable to me, but I'd let @wtgodbe take a look before merging.

[wtgodbe]

I'm a little wary of this given that it's something we haven't done before, but given that this is an OOB package & not a library that's in the SharedFx, it's probably not too risky. Approved modulo a few questions