Add missing abbreviations to abbreviations clause

For the purposes of the present document, the following abbreviations apply:

`CRA   Cyber Resilience Act`  

`CRL   Certificate Revocation List`  

`DNS   Domain Name Server`  

`EDR   Endpoint Detection and Response`  

`IAM   Identity and Access Management`  

`MITM  Machine-In-The-Middle`  

`OCSP  Online Certificate Status Protocol`  

`OS    Operating System`  

`OT    Operational Technology`  

`PKI   Public Key Infrastructure`  

`PSK   Pre-shared key`  

`RDPS  Remote Data Processing Solution`  

`SIEM  Security Information and Event Management`  

`TLS   Transport Layer Security`  

`TUF   The Update Framework`  

`UC    Use Case`