Migrate security doc from RST to Markdown

RST files are no longer correctly recognized by GitHub.

.github/SECURITY.md

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+# Security Policy
+
+**⚠️ Please do not file public GitHub issues for security
+vulnerabilities as they are open for everyone to see! ⚠️**
+
+We encourage responsible disclosure practices for security
+vulnerabilities.
+
+
+## Supported Versions
+
+Always update to the latest version of
+this Action to keep up with security patches.
+
+
+## Reporting a Vulnerability
+
+If you believe you've found a security-related bug, we
+prefer that you fill out a [vulnerability report on GitHub]
+directly.
+
+[vulnerability report on GitHub]:
+/pypa/gh-action-pypi-publish/security/advisories/new
+
+
+## Don't have a GitHub account?
+
+Alternatively, drop an email to
+``wk+gh-action-pypi-publish-security`` at ``sydorenko`` dot
+``org`` dot ``ua`` instead of filing a ticket or posting to
+_any_ public groups. We will try to assess the problem in
+timely manner and disclose it in a responsible way.

.github/SECURITY.rst

@@ -1,14 +0,0 @@
-Security Policy
----------------
-
-Supported Versions
-==================
-
-Always update to the latest version of
-this Action to keep up with security patches.
-
-Reporting a Vulnerability
-=========================
-
-Email to ``wk+gh-action-pypi-publish-security``
-at ``sydorenko`` dot ``org`` dot ``ua``.