Add external security standards back

[manics]

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⤴️ Summary

I removed Your TRE should adhere to one or more external security standards. in https://github.com/sa-tre/satre-specification/pull/269/files#r1345595654 as it doesn't fit in, and is arguably covered by
Internal Audit Process

However if we did want to explicity say something about adhering to external security standards (I'm undecided) then this is one suggestion for adding it to the IG pillar.

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