Update CI-settings to support ghc-9

Author: junjihashimoto

.github/workflows/ci.yml

@@ -70,9 +70,12 @@ jobs:
           # versions of our dependencies allowed by our upper bounds.
           #
           # TODO: switch back to "ghc: latest" once we support ghc-9.0
-          - name: newest
+          - name: oldest
             ghc: "8.10"
             os: ubuntu-latest
+          - name: newest
+            ghc: "9.0"
+            os: ubuntu-latest
 
     steps:
     - uses: actions/checkout@v2
@@ -120,6 +123,8 @@ jobs:
         include:
           - ghc: "8.10"
             os: ubuntu-latest
+          - ghc: "9.0"
+            os: ubuntu-latest
 
     steps:
     - uses: actions/checkout@v2

oldest-supported-lts.yaml

@@ -1,7 +1,7 @@
 # if this build fails and you need to bump the lts, remember
 # to also bump the lower bounds in package.yaml to match the
 # versions provided by that new lts!
-resolver: nightly-2020-12-10
+resolver: lts-18.3
 packages:
 - '.'
 

stack.yaml

@@ -1,7 +1,7 @@
-resolver: nightly-2020-12-10
+resolver: nightly-2021-07-22
 packages:
 - '.'
 
 extra-deps:
 - term-rewriting-0.4.0.2
-- union-find-array-0.1.0.2
+- union-find-array-0.1.0.3