docs: Update Go version as per the Readme.

[hristog]

This PR proposes an update to the Go version, specified in the contribution guidelines as per:

• The Go version, specified in the Readme file.
• The actual PR which updated the minimum supported Go versions, namely: https://github.com/btcsuite/btcd/pull/1753.

[coveralls]

Pull Request Test Coverage Report for Build 4147804037

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.002%) to 55.275%

---

Files with Coverage Reduction	New Missed Lines	%
btcutil/gcs/gcs.go	1	81.25%
connmgr/connmanager.go	1	86.27%
<!--	Total:	2		-->

Totals
Change from base Build 4073693760:	-0.002%
Covered Lines:	26638
Relevant Lines:	48192

---

💛 - Coveralls