Cache: Add awaits to memory cache rebuilds to fix race conditions

[nikolajlauridsen]

What does this PR fix?

This PR adds missing await keywords on async cache operations that were introduced in #17296.

Context

With the introduction of hybrid cache rebuilding, memory cache was added to cache refreshers in #17296. During this process, awaits were missed on async methods (see this example), meaning the run in parallel, which can't work.

This has caused race conditions which can result in various issues such as multiple open DataReaders and failure to acquire locks.

This issue becomes particularly apparent when adding an L2 cache like Redis, likely because hybridCache.RemoveAsync becomes truly asynchronous in that scenario. Additionally, this issue only affects SQL Server.

Testing

Due to the nature of race conditions, this is difficult to test reliably. However, the issue can be reproduced consistently by:

1. Installing using SQL Server
2. Creating and configuring Umbraco to use a Redis cache
3. Creating a document type
4. Creating content using the document type
5. Attempting to edit the document type
6. Failure occurs reliably

Redis cache configuration example:

Install Microsoft.Extensions.Caching.StackExchangeRedis

builder.Services.AddStackExchangeRedisCache(options =>
{
    options.Configuration = "{INSTANCE ID}";
    options.InstanceName = "{INSTANCE NAME}";
});

[Copilot]

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

[AndyButland]

I think I can review and approve this with just visual inspection, as it's clearly an existing oversight and a correct update.

[nikolajlauridsen]

Cherry picked for 17.0.1 🚀