Migrate from GitHub to Gitea

- Update all repository URLs to git.marketally.com
- Remove GitHub Actions workflows (not compatible with Gitea)
- Update documentation links in README, CONTRIBUTING, and CHANGELOG

composer.json

@@ -13,7 +13,7 @@
         "performance",
         "big-data"
     ],
-    "homepage": "https://github.com/sqrtspace/spacetime-php",
+    "homepage": "https://git.marketally.com/sqrtspace/sqrtspace-php",
     "license": "Apache-2.0",
     "authors": [
         {
@@ -85,11 +85,5 @@
         }
     },
     "minimum-stability": "stable",
-    "prefer-stable": true,
-    "repositories": [
-        {
-            "type": "composer",
-            "url": "https://nuget.pkg.github.com/sqrtspace/index.json"
-        }
-    ]
+    "prefer-stable": true
 }