Setting readOnlyRootFilesystem overrides any pre-existing securityContext

[olim7t]

If CassandraDatacenter.spec.readOnlyRootFilesystem is set, we build a completely new securityContext in construct_podtemplatespec.go:

	if dc.ReadOnlyFs() {
		cassContainer.SecurityContext = &corev1.SecurityContext{
			ReadOnlyRootFilesystem: ptr.To[bool](true),
		}
	}

However, this ignores the fact that other securityContext options might have been already provided via CassandraDatacenter.spec.podTemplateSpec.
We should amend the existing context (if present) instead of overriding it.

┆Issue is synchronized with this Jira Story by Unito
┆Issue Number: CASS-67