Bump CI Elixir/Erlang to fix warnings

There's at least one library that specifies that it requires Elixir 1.15
and setup-beam wasn't happy anymore with the old versions.

Author: fhunleth

.github/workflows/elixir.yml

@@ -14,8 +14,8 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        elixir: [1.11]
-        otp: [23.3]
+        elixir: [1.15]
+        otp: [26]
     steps:
     - name: checkout
       uses: actions/checkout@v4
@@ -51,8 +51,8 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        elixir: [1.11]
-        otp: [23.3]
+        elixir: [1.15]
+        otp: [26]
     steps:
     - uses: actions/checkout@v4
       with:
@@ -82,8 +82,8 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        elixir: [1.11]
-        otp: [23.3]
+        elixir: [1.15]
+        otp: [26]
     steps:
     - uses: actions/checkout@v4
       with: