[ANN] Sunset of legacy build system

The section under Questions for Team Discussion

Rewatch explicitly lists these as unsupported in UNSUPPORTED_TOP_LEVEL_FIELDS:

• ignored-dirs
• generators
• cut-generators
• pp-flags
• entries
• use-stdlib
• external-stdlib
• bs-external-includes
• reanalyze

The team needs to decide whether to:

• Remove: Drop support entirely in the major version
• Implement: Add support to rewatch before migration

Regardless of this list, if you cannot upgrade to the new build system: please let us know what is hindering you!