fix(language): moved british spellings to britishenglish file (@AnnaBuchholz) (monkeytypegame#6208)

### Description

I changed some words in English 25_k to American English spellings and I
added the alternate spellings in the britishenglish.json file. Some
words already had the American English spellings in the english_25k file
and the alternate spellings in the britishenglish.json file so I just
deleted the British versions.

### Checks

- [ ] Adding a language or a theme?
- [ ] If is a language, did you edit `_list.json`, `_groups.json` and
add `languages.json`?
  - [ ] If is a theme, did you add the theme.css?
- Also please add a screenshot of the theme, it would be extra awesome
if you do so!
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tag them below.
- [x] Make sure the PR title follows the Conventional Commits standard.
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parentheses at the end of the PR title.

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Author: AnnaBuchholz

frontend/static/languages/britishenglish.json

@@ -18,6 +18,8 @@
   ["likable", "likeable"],
   ["neutralize", "neutralise"],
   ["meter", "metre"],
+  ["meters", "metres"],
+  ["centimeter", "centimetre"],
   ["hurrah", "hooray"],
   ["multicolor", "multicolour"],
   ["actualization", "actualisation"],
@@ -523,6 +525,7 @@
   ["gray", "grey"],
   ["grays", "greys"],
   ["theater", "theatre"],
+  ["amphitheater", "amphitheatre"],
   ["center", "centre"],
   ["realize", "realise"],
   ["realizing", "realising"],

frontend/static/languages/english_25k.json

@@ -1590,7 +1590,6 @@
     "cervical",
     "abandonedly",
     "detestation",
-    "kilometre",
     "stadtholder",
     "pennies",
     "enumeration",
@@ -3749,7 +3748,6 @@
     "ornithologist",
     "Umbria",
     "selvage",
-    "millimetre",
     "sough",
     "gooseflesh",
     "pons",
@@ -6627,7 +6625,6 @@
     "latch",
     "epopt",
     "ballyhoo",
-    "nitre",
     "fallout",
     "stiffy",
     "distich",
@@ -6985,8 +6982,6 @@
     "saleable",
     "beefy",
     "meager",
-    "metre",
-    "masturbation",
     "scary",
     "caudillo",
     "tetrahedra",
@@ -8289,7 +8284,6 @@
     "trigger",
     "prosecute",
     "quested",
-    "concentre",
     "instrumentation",
     "comprehensible",
     "accommodating",
@@ -8743,7 +8737,6 @@
     "sciagraphy",
     "underclothes",
     "carpel",
-    "litre",
     "portugese",
     "dyer",
     "perspicuously",
@@ -12027,7 +12020,6 @@
     "lifetimes",
     "spotty",
     "sphincter",
-    "goitre",
     "abrasive",
     "organisms",
     "indisputable",
@@ -13720,7 +13712,6 @@
     "Nigeria",
     "Ludlow",
     "quarterage",
-    "masturbate",
     "hosannah",
     "Abbeville",
     "unconvinced",
@@ -15940,7 +15931,7 @@
     "proclivity",
     "asthenic",
     "tabular",
-    "amphitheatre",
+    "amphitheater",
     "sunni",
     "emanation",
     "smokable",
@@ -16164,7 +16155,6 @@
     "threesome",
     "condo",
     "pratt",
-    "masturbator",
     "confrontation",
     "coeus",
     "factitious",
@@ -17301,7 +17291,7 @@
     "polytheism",
     "evenly",
     "hance",
-    "centimetre",
+    "centimeter",
     "begum",
     "bermudian",
     "funnel",
@@ -20741,7 +20731,7 @@
     "fallibility",
     "encryption",
     "lechery",
-    "metres",
+    "meters",
     "leaper",
     "pinnacle",
     "pyrotechny",
@@ -21864,7 +21854,6 @@
     "desiccation",
     "unstrap",
     "kilt",
-    "saltpetre",
     "calamitous",
     "sumptuary",
     "netsplit",
@@ -22358,7 +22347,7 @@
     "bellies",
     "diocesan",
     "outmatch",
-    "accipitres",
+    "accipiters",
     "plowing",
     "eliminate",
     "azrael",