fix: store and read severity from linters in the cache

[ldez]

The severity from linters needs to be stored into the cache, if not, the severity between 2 calls is not consistent.

To test the PR

#!/bin/sh -e

## setup module
rm -rf testexample
mkdir testexample && cd $_
go mod init helloworld

## main.go
cat > main.go  <<EOF
// this is a very very very very very very very very very very very very very very very very very very very very very very long line.
package main

func main() {
	newDnsZone()
}

func newDnsZone() error {
	return nil
}
EOF

## .golangci.yml
cat > .golangci.yml  <<EOF
linters:
  disable-all: true
  enable:
    - revive
    - errcheck
    - lll

linters-settings:
  revive:
    ignore-generated-header: false
    severity: error
    confidence: 0.8
    rules:
      - name: var-naming
        severity: warning

severity:
  default-severity: error
  keep-linter-severity: true
  case-sensitive: true
  rules:
    - linters:
        - lll
      severity: warning
EOF

---

Before (i.e. v1.56.2)

$ golangci-lint cache clean

$ golangci-lint run --out-format json | jq  '.Issues[] | {linter: .FromLinter, text: .Text, severity: .Severity }' 
{
  "linter": "errcheck",
  "text": "Error return value is not checked",
  "severity": "error"
}
{
  "linter": "lll",
  "text": "line is 133 characters",
  "severity": "warning"
}
{
  "linter": "revive",
  "text": "var-naming: func newDnsZone should be newDNSZone",
  "severity": "error"
}

$ golangci-lint run --out-format json | jq  '.Issues[] | {linter: .FromLinter, text: .Text, severity: .Severity }'
{
  "linter": "errcheck",
  "text": "Error return value is not checked",
  "severity": "error"
}
{
  "linter": "lll",
  "text": "line is 133 characters",
  "severity": "warning"
}
{
  "linter": "revive",
  "text": "var-naming: func newDnsZone should be newDNSZone",
  "severity": "error"
}

The severity from linters is ignored.

After #4452

$ golangci-lint cache clean

$ ./golangci-lint run --out-format json | jq  '.Issues[] | {linter: .FromLinter, text: .Text, severity: .Severity }' 
{
  "linter": "errcheck",
  "text": "Error return value is not checked",
  "severity": "error"
}
{
  "linter": "lll",
  "text": "line is 133 characters",
  "severity": "warning"
}
{
  "linter": "revive",
  "text": "var-naming: func newDnsZone should be newDNSZone",
  "severity": "warning"
}

$ ./golangci-lint run --out-format json | jq  '.Issues[] | {linter: .FromLinter, text: .Text, severity: .Severity }'
{
  "linter": "errcheck",
  "text": "Error return value is not checked",
  "severity": "error"
}
{
  "linter": "lll",
  "text": "line is 133 characters",
  "severity": "warning"
}
{
  "linter": "revive",
  "text": "var-naming: func newDnsZone should be newDNSZone",
  "severity": "error"
}

The severity from linters is applied but the severity from var-naming has changed between the 2 calls because the severity is not inside the cache.

With this PR

$ golangci-lint cache clean

$ ./golangci-lint run --out-format json | jq  '.Issues[] | {linter: .FromLinter, text: .Text, severity: .Severity }'
{
  "linter": "errcheck",
  "text": "Error return value is not checked",
  "severity": "error"
}
{
  "linter": "lll",
  "text": "line is 133 characters",
  "severity": "warning"
}
{
  "linter": "revive",
  "text": "var-naming: func newDnsZone should be newDNSZone",
  "severity": "warning"
}

$ ./golangci-lint run --out-format json | jq  '.Issues[] | {linter: .FromLinter, text: .Text, severity: .Severity }'
{
  "linter": "errcheck",
  "text": "Error return value is not checked",
  "severity": "error"
}
{
  "linter": "lll",
  "text": "line is 133 characters",
  "severity": "warning"
}
{
  "linter": "revive",
  "text": "var-naming: func newDnsZone should be newDNSZone",
  "severity": "warning"
}

The severity stays consistent.

[ldez]

I cleaned to PR because I committed unrelated things by mistake.