template: Decrease max font size from 20px to 18px

Subjectively 20px is just too large
3 jobs for pgriffis/max-font-size in 2 minutes and 10 seconds
Status Name Job ID Coverage
  Check
passed flake8 #1469791

00:01:44

failed mypy #1469792
allowed to fail

00:01:59

passed reuse #1469793

00:02:10

 
Name Stage Failure
failed
mypy Check
gidocgen/gir/parser.py:984: error: Argument "get_type" to "GType" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:1006: error: Argument "name" to "Union" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:1006: error: Argument "symbol_prefix" to "Union" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:1006: error: Argument "ctype" to "Union" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:1008: error: Argument 1 to "set_version" of "GIRElement" has incompatible type "Optional[str]"; expected "str"
gidocgen/utils.py:823: error: Need type annotation for 'found_programs' (hint: "found_programs: Dict[<type>, <type>] = ...")
Found 211 errors in 3 files (checked 17 source files)
Cleaning up file based variables
ERROR: Job failed: exit code 1