feat: get last release with git tags

- Remove the `getLastRelease` plugin type
- Retrieve the last release based on Git tags
- Create the next release Git tag before calling the `publish` plugins

BREAKING CHANGE: Remove the `getLastRelease` plugin type

The `getLastRelease` plugins will not be called anymore.

BREAKING CHANGE: Git repository authentication is now mandatory

The Git authentication is now mandatory and must be set via `GH_TOKEN`, `GITHUB_TOKEN`,  `GL_TOKEN`, `GITLAB_TOKEN` or `GIT_CREDENTIALS` as described in [CI configuration](https://github.com/semantic-release/semantic-release/blob/caribou/docs/usage/ci-configuration.md#authentication).

lib/plugins/definitions.js

@@ -1,5 +1,4 @@
-const {isString, isObject, isFunction, isArray} = require('lodash');
-const semver = require('semver');
+const {isString, isFunction, isArray} = require('lodash');
 
 const RELEASE_TYPE = ['major', 'premajor', 'minor', 'preminor', 'patch', 'prepatch', 'prerelease'];
 const validatePluginConfig = conf => isString(conf) || isString(conf.path) || isFunction(conf);
@@ -13,22 +12,6 @@ module.exports = {
         'The "verifyConditions" plugin, if defined, must be a single or an array of plugins definition. A plugin definition is either a string or an object with a path property.',
     },
   },
-  getLastRelease: {
-    default: '@semantic-release/npm',
-    config: {
-      validator: conf => Boolean(conf) && validatePluginConfig(conf),
-      message:
-        'The "getLastRelease" plugin is mandatory, and must be a single plugin definition. A plugin definition is either a string or an object with a path property.',
-    },
-    output: {
-      validator: output =>
-        !output ||
-        (isObject(output) && !output.version) ||
-        (isString(output.version) && Boolean(semver.valid(semver.clean(output.version))) && Boolean(output.gitHead)),
-      message:
-        'The "getLastRelease" plugin output if defined, must be an object with a valid semver version in the "version" property and the corresponding git reference in "gitHead" property.',
-    },
-  },
   analyzeCommits: {
     default: '@semantic-release/commit-analyzer',
     config: {