[securemail] Upgrade Composer dependencies to fix PHP8 issue

- Upgrading phpseclib/phpseclib (2.0.4 => 2.0.34)
- Upgrading singpolyma/openpgp-php (0.3.0 => 0.5.0)

securemail/vendor/singpolyma/openpgp-php/examples/README.md

@@ -0,0 +1,22 @@
+OpenPGP.php Examples
+====================
+
+The scripts in this folder show how to use this library to perform various tasks
+such as [generating a new key](keygen.php), [signing a message](sign.php), and
+[verifying a message](verify.php) that has been signed.
+
+To use these examples, make sure [`phpseclib`](http://phpseclib.sourceforge.net/) is available. You can install it
+using [Composer](https://getcomposer.org/):
+
+```sh
+git clone https://github.com/singpolyma/openpgp-php.git # Clone the repository.
+cd openpgp-php
+composer install # Use Composer to install the requirements.
+```
+
+Once Composer has installed the requirements, run the examples using PHP:
+
+```sh
+# Generate a new OpenPGP key; see the `keygen.php` file for parameters.
+php ./examples/keygen.php > mykey.gpg
+```