Dropped support for Node v18.

Cypress no longer supports this [1] and I currently don't maintain
different version lines of this library. Hence, this seems like the path
of least resistance.

[1] https://docs.cypress.io/app/references/changelog#15.0.0

Author: badeball

.github/workflows/build.yml

@@ -31,7 +31,7 @@ jobs:
     steps:
       - uses: actions/setup-node@v4
         with:
-          node-version: 18
+          node-version: 20
       - name: Checkout
         uses: actions/checkout@v4
       - name: Cache NPM modules
@@ -71,7 +71,7 @@ jobs:
     steps:
       - uses: actions/setup-node@v4
         with:
-          node-version: 18
+          node-version: 20
       - name: Checkout
         uses: actions/checkout@v4
       - name: Cache NPM modules

.github/workflows/examples-branch.yml

@@ -71,7 +71,7 @@ jobs:
     steps:
       - uses: actions/setup-node@v4
         with:
-          node-version: 18
+          node-version: 20
       - name: Checkout
         uses: actions/checkout@v4
       - name: Cache NPM modules

.github/workflows/examples-master.yml

@@ -36,7 +36,7 @@ jobs:
     steps:
       - uses: actions/setup-node@v4
         with:
-          node-version: 18
+          node-version: 20
       - name: Checkout
         uses: actions/checkout@v4
       - name: Cache NPM modules

CHANGELOG.md

@@ -8,6 +8,8 @@ Breaking changes:
 
 - Dropped support for Cypress v11.
 
+- Dropped support for Node v18.
+
 Other changes:
 
 - Added support for Cypress v15, fixes [#1312](https://github.com/badeball/cypress-cucumber-preprocessor/issues/1312).