DOC: Fix improper single backticks in whatsnew
Noticed a few places in whatsnew where items are only surrounded by single backticks, which appears to cause them to render as italic.  For the most part I replaced these with double backticks, e.g. <code>\`Series\`</code> -> <code>\`\`Series\`\`</code>, or removed the backticks if they didn't seem appropriate.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17916?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@5687f9e`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17916/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/17916?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #17916   +/-   ##
=========================================
  Coverage          ?   91.22%           
=========================================
  Files             ?      163           
  Lines             ?    50107           
  Branches          ?        0           
=========================================
  Hits              ?    45710           
  Misses            ?     4397           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.03% <ø> (?)` | |
| #single | `40.32% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17916?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17916?src=pr&el=footer). Last update [5687f9e...fbffdb6](https://codecov.io/gh/pandas-dev/pandas/pull/17916?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17916?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@5687f9e`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17916/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17916?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #17916   +/-   ##
=========================================
  Coverage          ?   91.22%           
=========================================
  Files             ?      163           
  Lines             ?    50107           
  Branches          ?        0           
=========================================
  Hits              ?    45710           
  Misses            ?     4397           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.03% <ø> (?)` | |
| #single | `40.31% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17916?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17916?src=pr&el=footer). Last update [5687f9e...fbffdb6](https://codecov.io/gh/pandas-dev/pandas/pull/17916?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks!