Refactor how version numbers are specified after the introduction of installer stages

Now that [every Dockerfile is utilizing an installer stage](https://github.com/dotnet/dotnet-docker/pull/6245), consider changing how the .NET version numbers are being handled.

Current Pattern (.NET version is being specified in multiple places):

```Dockerfile
ARG REPO=mcr.microsoft.com/dotnet/runtime-deps

# Installer image
FROM amd64/buildpack-deps:noble-curl AS installer

# Retrieve .NET Runtime
RUN dotnet_version=10.0.0-preview.1.25080.5 \
&& curl -fSL --output dotnet.tar.gz https://ci.dot.net/public/Runtime/$dotnet_version/dotnet-runtime-$dotnet_version-linux-x64.tar.gz \
...

# .NET runtime image
FROM $REPO:10.0.0-preview.1-noble-amd64

# .NET Runtime version
ENV DOTNET_VERSION=10.0.0-preview.1.25080.5
```

Proposal (Central the .NET version number):

```Dockerfile
ARG REPO=mcr.microsoft.com/dotnet/runtime-deps
ARG DOTNET_VERSION=10.0.0-preview.1.25080.5

# Installer image
FROM amd64/buildpack-deps:noble-curl AS installer
ARG DOTNET_VERSION

# Retrieve .NET Runtime
RUN curl -fSL --output dotnet.tar.gz https://ci.dot.net/public/Runtime/${DOTNET_VERSION}/dotnet-runtime-$dotnet_version-linux-x64.tar.gz \
...

# .NET runtime image
FROM $REPO:10.0.0-preview.1-noble-amd64
ARG DOTNET_VERSION

# .NET Runtime version
ENV DOTNET_VERSION=${DOTNET_VERSION}
```

Improves readability and maintainability of the Dockerfiles which is a benefit to the maintainers of this repo as well as customers who copy the Dockerfiles which is sometimes necessary when they can't make use of the .NET images themselves.