This commit standardizes the SSL settings by bumping the logstash-mixin-http_client version to >= 7.3.0, which:

- Adds standardized SSL settings and deprecates their non-standard counterparts. Deprecated settings will continue to work, and will provide pipeline maintainers with guidance toward using their standardized counterparts
- Adds new ssl_truststore_path, ssl_truststore_password, and ssl_truststore_type settings for configuring SSL-trust using a PKCS-12 or JKS trust store, deprecating their truststore, truststore_password, and truststore_type counterparts.
- Adds new ssl_certificate_authorities setting for configuring SSL-trust using a PEM-formatted list of certificate authorities, deprecating its cacert counterpart.
Adds new ssl_keystore_path, ssl_keystore_password, and ssl_keystore_type settings for configuring SSL-identity using a PKCS-12 or JKS key store, deprecating their keystore, keystore_password, and keystore_type counterparts.
- Adds new ssl_certificate and ssl_key settings for configuring SSL-identity using a PEM-formatted certificate/key pair, deprecating their client_cert and client_key counterparts.
- Adds the ssl_cipher_suites option

This commit added the `ssl_enabled` option by bumping the `logstash-mixin-http_client` to >= 7.4.0

Unit tests were failing due to a new requirement to include the `rackup`
gem. This commit includes the version of `rackup` prior to the Fiber based rework,
as that was causing test failures.

This commit also ensures that the request body is rewound before being read in tests.

* Mark deprecated SSL settings as obsolete

This commit marks the following SSL settings as obsolete:

'cacert', 'client_cert', 'client_key', 'keystore', 'truststore',
'keystore_password', 'truststore_password', 'keystore_type' and
'truststore_type'.

…patch-1

Docs: clarify that `ssl_certificate_authorities` takes at-most-one value