Replace size with linewidth

[ndporter]

Replace size with linewidth for geom_smooth() example. Using size for lines was deprecated per this error message:

> ggplot(data=gapminder,
+        mapping=aes(x=gdpPercap,
+                    y=lifeExp)) +
+   geom_point(alpha=0.5) +
+   scale_x_log10() +
+   geom_smooth(method='lm', size=1.5)
`geom_smooth()` using formula = 'y ~ x'
Warning message:
Using `size` aesthetic for lines was deprecated in ggplot2 3.4.0.
ℹ Please use `linewidth` instead.
This warning is displayed once every 8 hours.
Call `lifecycle::last_lifecycle_warnings()` to see where this warning was generated. 

Change throughout the lesson and additionally:

1. Adjust language in explanations to match
2. Add hint to challenge clarifying size is point equivalent of linewidth.
3. Add some additional explanation of difference between aesthetics with vs without mappings.

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[github-actions]

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Rendered Changes

🔍 Inspect the changes: https://github.com/swcarpentry/r-novice-gapminder/compare/md-outputs..md-outputs-PR-890

The following changes were observed in the rendered markdown documents:

 01-rstudio-intro.md |    2 +-
 02-project-intro.md |    2 +-
 03-seeking-help.md  |    6 +-
 08-plot-ggplot2.md  |   23 +-
 config.yaml (gone)  |   95 -----
 md5sum.txt          |   50 +--
 renv.lock (gone)    | 1021 ---------------------------------------------------
 7 files changed, 37 insertions(+), 1162 deletions(-)

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⏱️ Updated at 2024-05-16 13:15:02 +0000

[skanwal]

Hi @ndporter - thanks so much for addressing this issue.
The changes look good to me.

[matthieu-bruneaux]

Thanks @ndporter and @skanwal! This looks good to me too.