Set MSRV to 1.78

1.78.0 is the first version of cargo that supports lock version
v4 (the version of our Cargo.lock file).

Author: anoadragon453

Cargo.toml

@@ -31,6 +31,7 @@ tikv-jemallocator = "0.6.0"
 [package]
 name = "synapse_compress_state"
 description = "A tool to compress some state in a Synapse instance's database"
+rust-version = "1.78"
 authors = ["Erik Johnston"]
 version.workspace = true
 edition.workspace = true