Enable golangci linter gomnd

[xoxys]

Split out from #2960

Some remaining issues where I would like to get some input:

1. Why is != 2 correct here? Would like to document what 2 means and why it is the intended value.

server/model/repo.go:69:19: mnd: Magic number: 2, in <condition> detected (gomnd)
	if len(parts) != 2 {
	                 ^

2. Why is <= 3 correct here? Would like to document what 3 means and why it is the intended value.

pipeline/shared/replace_secrets.go:23:18: mnd: Magic number: 3, in <condition> detected (gomnd)
		if len(old) <= 3 {
		               ^

3. Why is == 2 correct here? Would like to document what 2 means and why it is the intended value.

cli/exec/exec.go:246:18: mnd: Magic number: 2, in <condition> detected (gomnd)
	if len(base) == 2 {
	                ^

4. Why 8 bytes? Would like to document why it is the intended value.

pipeline/multipart/reader.go:55:21: mnd: Magic number: 8, in <argument> detected (gomnd)
	out, _ := buf.Peek(8)
	                   ^

5. Why is < 2 correct here? Would like to document what 2 means and why it is the intended value.

pipeline/backend/docker/convert.go:135:19: mnd: Magic number: 2, in <condition> detected (gomnd)
		if len(parts) < 2 {
		                ^

[woodpecker-bot]

Deployment of preview was torn down

[qwerty287]

1. Why is != 2 correct here? Would like to document what 2 means and why it is the intended value.

This func parses the owner/repo strings. If this is != 2, it's invalid. (I wonder how this works with gitlab subgroups how.)

2. Why is <= 3 correct here? Would like to document what 3 means and why it is the intended value.

See #2680

[qwerty287]

3. Why is == 2 correct here? Would like to document what 2 means and why it is the intended value.

The func returns windows volume name like C: consisting of one char + colon = two chars. https://pkg.go.dev/path/filepath#VolumeName

4. Why 8 bytes? Would like to document why it is the intended value.

If you look a few lines below, the output array is compared to the string PIPELINE (I don't know why). Pipeline has 8 chars.

5. Why is < 2 correct here? Would like to document what 2 means and why it is the intended value.

Func splits docker volumes. They consist of two paths separated by a colon, e.g. /path/xyz:/path/x/y/z. Having more than one : would be incorrect syntax.