[branch-52] Remove dependency on rust_decimal, remove ignore of RUSTSEC-2026-0001 (#19666) (#19686)

## Which issue does this PR close?

- part of #18566

## Rationale for this change

Let's resolve the audit workflow on branch-52 by removing our dependency
on the `rust_decimal` library, per @xudong963 's request here
#19666 (review)

## What changes are included in this PR?

- Backport  #19666 to branch-52

## Are these changes tested?

By CI

## Are there any user-facing changes?
No, this is an internal dev tool change

Author: alamb

.github/workflows/audit.yml

@@ -46,4 +46,6 @@ jobs:
         with:
           tool: cargo-audit
       - name: Run audit check
+        # Note: you can ignore specific RUSTSEC issues using the `--ignore` flag ,for example:
+        # run: cargo audit --ignore RUSTSEC-2026-0001
         run: cargo audit

Cargo.lock

@@ -55,9 +55,7 @@ indicatif = "0.18"
 itertools = { workspace = true }
 log = { workspace = true }
 object_store = { workspace = true }
-postgres-protocol = { version = "0.6.7", optional = true }
 postgres-types = { version = "0.2.11", features = ["derive", "with-chrono-0_4"], optional = true }
-rust_decimal = { version = "1.38.0", features = ["tokio-pg"] }
 # When updating the following dependency verify that sqlite test file regeneration works correctly
 # by running the regenerate_sqlite_files.sh script.
 sqllogictest = "0.28.4"
@@ -75,7 +73,6 @@ postgres = [
     "bytes",
     "chrono",
     "postgres-types",
-    "postgres-protocol",
     "testcontainers-modules",
     "tokio-postgres",
 ]