Add pragma to suppress all obsoletion warnings.

Context: dotnet/runtime#101738

Source-generated files that use obsolete user-provided APIs will have obsoletion warnings themselves. The standard warning code can be suppressed at the start of the generated file, but if the API is obsoleted with a custom diagnostic ID, the warning cannot be easily suppressed just for that file.

To solve this, I propose to introduce a `#pragma warnings disable obsolete` directive that suppresses all obsoletetion warnings regardless of ID.

c.c. @RikkiGibson

Start by reading the proposal and its linked context, dotnet/runtime#101738, to understand the source-generated-file scenario and custom obsoletion diagnostic IDs. Then locate Roslyn's pragma-warning parsing and obsoletion diagnostic handling; done means the proposed directive suppresses obsoletion warnings regardless of diagnostic ID while preserving existing warning behavior.

Written by the indexing model from the issue text.