Bump minimum Go version to 1.21

Author: mholt

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       matrix:
         os: [ ubuntu-latest, macos-latest, windows-latest ]
-        go: [ '1.19', '1.20' ]
+        go: [ '1.21', '1.22' ]
 
     runs-on: ${{ matrix.os }}
 

README.md

@@ -125,6 +125,7 @@ CertMagic - Automatic HTTPS using Let's Encrypt
 4. Persistent storage
 	- Typically the local file system (default)
 	- Other integrations available/possible
+5. Go 1.21 or newer
 
 **_Before using this library, your domain names MUST be pointed (A/AAAA records) at your server (unless you use the DNS challenge)!_**