refactor!: Revert the bump of MSRV to 1.75 (#3952)

Revert "refactor!: Bump MSRV to 1.75 (#3851)"

This reverts commit 3edaa8f.

.github/workflows/ci.yml

@@ -161,8 +161,8 @@ jobs:
   check_msrv:
     runs-on: ubuntu-latest
     env:
-      # OpenDAL's MSRV is 1.75.
-      OPENDAL_MSRV: "1.75"
+      # OpenDAL's MSRV is 1.67.
+      OPENDAL_MSRV: "1.67"
     steps:
       - uses: actions/checkout@v4
       - name: Setup msrv of rust

Cargo.toml

@@ -54,7 +54,7 @@ edition = "2021"
 homepage = "https://opendal.apache.org/"
 license = "Apache-2.0"
 repository = "https://github.com/apache/incubator-opendal"
-rust-version = "1.75"
+rust-version = "1.67"
 version = "0.44.1"
 
 [workspace.dependencies]