Upgrade gcc to v15

[am11]

Release: https://gcc.gnu.org/gcc-15/

[am11]

@mthalman, it requires library/gcc:15-bookworm uploaded to dotnet ACR.

[mthalman]

The path in the CODEOWNERS file needs to be updated:

dotnet-buildtools-prereqs-docker/CODEOWNERS

Line 35 in e020122

src/debian/12/gcc14/amd64/ @dotnet/runtime-infrastructure @dotnet/dotnet-docker-reviewers

That should be changed to a wildcard for the version so we don't have to keep updating it.

[mthalman]

@am11 - Will you be updating all dotnet repos that referenced the old v14 tag?

[am11]

Already done dotnet/runtime#115128. :)

[mthalman]

It's also referenced in the sdk repo:

https://github.com/dotnet/sdk/blob/a1da6b61d1552406d1a67cbbc19b25880f4b8308/.vsts-pr.yml#L34-L35

https://github.com/dotnet/sdk/blob/a1da6b61d1552406d1a67cbbc19b25880f4b8308/.vsts-ci.yml#L71-L72

[am11]

It was a wrong dependency added in dotnet/sdk@df4aa4c. gcc image was introduced to validate "runtime builds with gcc". Debian packaging use-case in SDK should be its own image. It doesn't require gcc, let alone the latest version.