refactor: remove legacy v1 schema migration logic #4862

[neerajvipparla]

Changes for #4862 — Remove schema V1 support and metadata v2 migration

What: PR #4861 added a one-shot migration that, on node startup, detected whether the schema in the metadata store was still in the old V1 format and re-wrote it as V2. That migration code was explicitly marked // todo(azmy): Remove in Restate v1.8. This PR does that cleanup, and also removes the V1 schema deserialization path that the migration depended on.

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Deleted

• crates/core/src/metadata/metadata_migrations.rs — the transit migrate_metadata function that read-modify-wrote the schema to force a V1→V2 re-serialization. No longer needed: any node that ran ≥1.7 has already migrated.

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Modified

crates/core/src/lib.rs / crates/core/src/metadata.rs
Removed the mod metadata_migrations declaration and its pub use migrate_metadata re-export from both the inner and outer module.

crates/node/src/lib.rs
Removed the migrate_metadata import and the call to it inside Node::start. The startup sequence no longer performs a schema migration.

crates/types/src/schema/metadata/mod.rs
Removed the three additions that PR #4861 made to Schema:

• legacy_v1: bool field (used as a sentinel to know migration was needed)
• is_legacy_v1() accessor
• touch() helper (bumped the version to force a metadata-store write)

crates/types/src/schema/metadata/serde_hacks.rs (largest change — ~1460 → 85 lines)
Removed all V1 schema support that predated PR #4861:

• mod v1_data_model — the old on-disk types (ServiceSchemas, ServiceLocation, DeploymentSchemas, HandlerSchemas, InvocationTargetMetadata and their inner serde compat shims)
• mod conversions — V1Schemas/V2Schemas structs and into_v2/into_v1 conversion logic
• The services and deployments (V1) fields from the intermediate serde Schema struct
• The V1 deserialization branch (else if let (Some(services), Some(deployments))) in From for super::Schema
• Both integration tests (old_to_new, v2_to_v1_to_schema) that exercised V1 round-trips

The From for super::Schema impl now expects deployments_v2 to be present — any attempt to load V1 data against v1.8+ will produce a clear error message directing the user to upgrade via v1.7 first.