build: Bump MSRV to 1.88

This is necessary to use the let-chains feature in a
follow-up. After upgrading to Rust edition 2024, clippy
wants to collapse various if's with let-chains.

Update image to 20250815-0 since MSRV in Dockerfile is updated.

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
On-behalf-of: SAP philipp.schuster@sap.com

.github/workflows/release.yaml

@@ -45,7 +45,7 @@ jobs:
           target: ${{ matrix.platform.target }}
           args: ${{ matrix.platform.args  }}
           strip: true
-          toolchain: "1.87.0"
+          toolchain: "1.88.0"
       - name: Copy Release Binaries
         if: github.event_name == 'create' && github.event.ref_type == 'tag'
         shell: bash