refactor: lake: deprecate -U (#6798)

This PR deprecates the `-U` shorthand for the `--update` option.

It is likely the `-U` option will be used for something different in the
future, so deprecating it now seems wise.

src/lake/Lake/CLI/Help.lean

@@ -46,7 +46,7 @@ BASIC OPTIONS:
   -K key[=value]        set the configuration file option named key
   --old                 only rebuild modified modules (ignore transitive deps)
   --rehash, -H          hash all files for traces (do not trust `.hash` files)
-  --update, -U          update dependencies on load (e.g., before a build)
+  --update              update dependencies on load (e.g., before a build)
   --packages=file       JSON file of package entries that override the manifest
   --reconfigure, -R     elaborate configuration files instead of using OLeans
   --keep-toolchain      do not update toolchain on workspace update

src/lake/Lake/CLI/Main.lean

@@ -172,7 +172,9 @@ def lakeShortOption : (opt : Char) → CliM PUnit
 | 'd' => do let rootDir ← takeOptArg "-d" "path"; modifyThe LakeOptions ({· with rootDir})
 | 'f' => do let configFile ← takeOptArg "-f" "path"; modifyThe LakeOptions ({· with configFile})
 | 'K' => do setConfigOpt <| ← takeOptArg "-K" "key-value pair"
-| 'U' => modifyThe LakeOptions ({· with updateDeps := true})
+| 'U' => do
+  logWarning "the '-U' shorthand for '--update' is deprecated"
+  modifyThe LakeOptions ({· with updateDeps := true})
 | 'R' => modifyThe LakeOptions ({· with reconfigure := true})
 | 'h' => modifyThe LakeOptions ({· with wantsHelp := true})
 | 'H' => modifyThe LakeOptions ({· with trustHash := false})