Project governance: introduce "emeritus" member status

### Describe the bug

Currently, there is no term for Fluentd maintainers.

More contributor involved, it tend to hard to call for voting in a timely manner which is described in [Voting](https://github.com/fluent/fluentd/blob/master/GOVERNANCE.md#voting)
because it requires 2/3 majority.

This situation is caused because inactive members and active members are treated as "same" individuals.

### To Reproduce

N/A

### Expected behavior

Introduce "emeritus" maintainer status

"emeritus" means:

* Inactive members (no commit, no community activity) for SPECIFIC TERM(T.B.D.)
* Marked inactive members "emeritus" and add such status will be described "emeritus" section in MAINTAINERS.md (do not remove because I honor their achievements)
* No voting qualification

https://github.com/fluent/fluentd/blob/master/GOVERNANCE.md#voting should be revised such as:

```
New maintainers are proposed by an existing maintainer and are elected by a 2/3 majority organization vote.

Maintainers can be resigned or removed by a 2/3 majority organization vote.

Inactive or resigned members will be treated as having "emeritus" status. "emeritus" status of maintainers don't have a ballot to vote.
Inactive means: no commitment or no community activity for 5 years.
```

NOTE: above sentence is just an draft.

### Your Environment

```markdown
N/A
```

### Your Configuration

```apache
N/A
```

### Your Error Log

```shell
N/A
```

### Additional context

Apache project introduces "emeritus"
ref. https://www.apache.org/dev/pmc.html#emeritus