Support TLS 1.3

[julianbrost]

This PR adds support for TLS 1.3.

Unfortunately, there's no method value for ssl::context that specifies TLS 1.2 or newer but only any TLS version, i.e. TLS 1.0 or newer (https://www.boost.org/doc/libs/1_75_0/doc/html/boost_asio/reference/ssl__context/method.html). That's not too much of an issue though as the call to SetupSslContext() will disable TLS 1.0 and 1.1.

fixes #8715

[julianbrost]

Noteworthy limitation of this PR: There is currently no way to configure the set of accepted cipher suites for TLS 1.3. Icinga currently uses SSL_CTX_set_cipher_list() which only allows to configure ciphers for TLS 1.2 and lower, for TLS 1.3 there is SSL_CTX_set_ciphersuites() but there seems to be no API that supports both at the same time.

So there are a few options now:

1. Say that this is fine as TLS 1.3 only has modern crypto
2. Add a second config attribute for TLS 1.3 cipher suites
3. Attempt to figure out which cipher Name goes into which function and try to split the value of existing config attribute (I don't like this option as the old cipher strings are quite complex and we'd have to put in quite some parsing effort not to break them)
4. Do something like nginx did and add an attribute that uses the SSL_CONF_cmd API of OpenSSL to allow setting all kind of options.

[lippserd]

1. Say that this is fine as TLS 1.3 only has modern crypto

I would do it that way.