Update blog post about package lock files to mention recommendation for SDK version pinning

[Frulfump]

https://devblogs.microsoft.com/dotnet/enable-repeatable-package-restores-using-a-lock-file/

Rationale
See comment from Microsoft(Chet) here dotnet/sdk#48795 (comment)

Our current recommendation for users that use lock files is to also lock their SDK versions via global.json with no rollforward, so they(sic) their entire toolchain stays in lockstep.

Discovery of this recommendation is currently hard (recently improved by docs updated due to below issues)

See related docs issue
dotnet/docs#53347
dotnet/docs#53517
NuGet/Home#14867