Update .NET SDK version in README.md

[mapaba79]

This PR updates the README to point to the correct .NET SDK version.

[nGoline]

ACK

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
NLightning.Application	30%	22%	258	❌
NLightning.Bolt11	89%	84%	504	✔
NLightning.Bolt11.Blazor	0%	0%	434	❌
NLightning.Domain	73%	50%	1571	➖
NLightning.Infrastructure	56%	47%	1004	➖
NLightning.Infrastructure.Bitcoin	56%	50%	467	➖
NLightning.Infrastructure.Persistence	0%	0%	156	❌
NLightning.Infrastructure.Persistence.Postgres	0%	100%	7	❌
NLightning.Infrastructure.Persistence.Sqlite	0%	100%	4	❌
NLightning.Infrastructure.Persistence.SqlServer	0%	100%	7	❌
NLightning.Infrastructure.Repositories	0%	0%	474	❌
NLightning.Infrastructure.Serialization	67%	51%	861	➖
NLightning.Node	10%	12%	215	❌
NLightning.Tests.Utils	84%	50%	19	✔
Summary	42% (6654 / 15905)	43% (1756 / 4115)	5981	❌