docs: Modify prerequisite go version

[sanposhiho]

What type of PR is this?

What this PR does / why we need it:

We're on golang v1.22.x, and this PR modifies the prerequisite based on it.
https://github.com/envoyproxy/gateway/blob/main/go.mod#L3

FYI, actually maybe we can just mention 1.21+ due to the following improvement, instead of mention the specific version, which is troublesome as we have to update the page every time we upgrade go version.

After installing Go 1.21, you’ll never have to manually download and install a Go toolchain again. The go command can do it for you.
https://go.dev/blog/toolchain

Which issue(s) this PR fixes:

Fixes #

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.79%. Comparing base (51c6eb4) to head (6f3e33b).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3711      +/-   ##
==========================================
- Coverage   68.81%   68.79%   -0.02%     
==========================================
  Files         175      175              
  Lines       21525    21525              
==========================================
- Hits        14812    14808       -4     
- Misses       5636     5638       +2     
- Partials     1077     1079       +2     

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