Fix issues indexer document mapping (#25619)

Fix regression of #5363 (so long ago).

The old code definded a document mapping for `issueIndexerDocType`, and
assigned it to `BleveIndexerData` as its type. (`BleveIndexerData` has
been renamed to `IndexerData` in #25174, but nothing more.) But the old
code never used `BleveIndexerData`, it wrote the index with an anonymous
struct type. Nonetheless, bleve would use the default auto-mapping for
struct it didn't know, so the indexer still worked. This means the
custom document mapping was always dead code.

The custom document mapping is not useless, it can reduce index storage,
this PR brings it back and disable default mapping to prevent it from
happening again. Since `IndexerData`(`BleveIndexerData`) has JSON tags,
and bleve uses them first, so we should use `repo_id` as the field name
instead of `RepoID`.

I did a test to compare the storage size before and after this, with
about 3k real comments that were migrated from some public repos.

Before:

```text
[ 160] .
├── [ 42] index_meta.json
├── [ 13] rupture_meta.json
└── [ 128] store
├── [6.9M] 00000000005d.zap
└── [256K] root.bolt
```

After:

```text
[ 160] .
├── [ 42] index_meta.json
├── [ 13] rupture_meta.json
└── [ 128] store
├── [3.5M] 000000000065.zap
└── [256K] root.bolt
```

It saves about half the storage space.

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Co-authored-by: Giteabot <teabot@gitea.io>