Compile errors with golang 1.18. Need to stop using atomic.Pointer or change README.md so minimal golang required is updated. Please note requiring golang upgrade would "break" Ubuntu LTS.

[J-A-M-E-5]

Compile error:-
core/blockchain.go:202:27: undefined: atomic.Pointer

Same thing happened a few weeks ago here: #26679

Users of Ubuntu 22.04 LTS now have to compile a new version of go as the repo version is 1.18. Or these changes need to be backed out which is what happened 2 weeks ago. Can your automated builds not use 1.18 to spot these errors, until you decide to no longer support 1.18?

[J-A-M-E-5]

This appears to be the change that broke it: #26777

Looking more closely at change and comments @karalabe is aware that this breaks go1.18 and comments that effectively go1.18 is no longer supported. Therefor I suggest README.md is changed to reflect this.

[karalabe]

Bumped the minimum version to Go 1.19 so there's no more confusion.