docs: add links to more audits in SECURITY.md (#23482)

Author: fredrik0x

SECURITY.md

@@ -12,6 +12,8 @@ Audit reports are published in the `docs` folder: https://github.com/ethereum/go
 | ------- | ------- | ----------- |
 | `geth` | 20170425 | [pdf](https://github.com/ethereum/go-ethereum/blob/master/docs/audits/2017-04-25_Geth-audit_Truesec.pdf) |
 | `clef` | 20180914 | [pdf](https://github.com/ethereum/go-ethereum/blob/master/docs/audits/2018-09-14_Clef-audit_NCC.pdf) |
+| `Discv5` | 20191015 | [pdf](https://github.com/ethereum/go-ethereum/blob/master/docs/audits/2019-10-15_Discv5_audit_LeastAuthority.pdf) |
+| `Discv5` | 20200124 | [pdf](https://github.com/ethereum/go-ethereum/blob/master/docs/audits/2020-01-24_DiscV5_audit_Cure53.pdf) |
 
 ## Reporting a Vulnerability